IPv4 Addressing and Subnetting. G. Gianini

Transcription

1 IPv4 Addressing and Subnetting G. Gianini

2 Summary Addressing basics in IPv4 Limits and problems Fixed Mask Subnetting Variable Lenght Subnet Masking A look at CIDR and IPv6

3 IPv4 Addressing basics

4 IPv4 Header

5 Dotted-decimal notation The IPv4 address space consists of a 32 bit field, or the equivalent of some 4.5 billion values

6

7 IP address classes

8 The three classes we focus on

9 Some special addresses

10 Zero and All One Host Numbers The values 0 and -1 (all ones) have always special meanings when used in Host Numbers : The value zero means this host The value -1 is used as a broadcast address to mean all hosts of the indicated network As a consequence if n bits are reserved for the host addressing only 2^n -2 different hosts can be given an address.

11 Zero Network Numbers In Network numbers of class A,B or C, the zero network number has a special meaning in the three following cases: 0.x.x.x means this network within a class A network 0.0.x.x means this network within a class B network x means this network within a class C network

12 The all zero address As a consequence of the zero rules for nets and host means this host

13 The all Ones Network Number By convention indicates a broadcast on the local network

14 How many nets? How many hosts? Class A Networks (/8 Prefixes) Class B Networks (/16 Prefixes) Class D Networks (/24 Prefixes)

15 A B C _

16 2^31 IP addresses (=2,147,483,648) distributed over 2^7 possible network addresses each with 2^24 hosts (16,777,216) From those figures one must subtract special addresses as mentioned above

17 2^30 IP addresses (=1,073,741,824) distributed over 2^14 possible network addresses each with 2^16 hosts (=65,536) From those figures one must subtract special addresses as mentioned above

18 2^29 IP addresses (=536,870,912) distributed over 2^21 network addresses (=2,097,152) each with 2^8 hosts (=256) From those figures one must subtract special addresses as mentioned above

19 Number of networks (log2 n) A B C Network size (log2 n.hosts) Number of Networks Network size

20 Problems

21 What if reality does not fit the theory? Think of systems of objects of different sizes (such as vehicles) and of their distribution (if a parking lot doesn t fit the actual vehicles distribution we are unhappy) Think of different case studies of object naming grouped objects: people addresses in cities (mail addressing) telephone numbers car plates in provinces and states computer addresses and organizations

22 Unforeseen Limitations to Classful Addressing The original designers never envisioned that the Internet would grow into what it has become today. (Unforseen developements which clash against the insufficient allocation of a resource are quite common in many areas: think of Y2K) Many of the problems that the Internet is facing today can be traced back to the early decisions that were made during its formative years.

23 Depletion of address space During the early days of the Internet, the seemingly unlimited address space allowed IP addresses to be allocated to an organization based on its request rather than its actual need. As a result, addresses were freely assigned to those who asked for them without concerns about the eventual depletion of the IP address space. The decision to standardize on a 32-bit address space meant that there were only 2^32 = (4,294,967,296) IPv4 addresses available. A decision to support a slightly larger address space would have exponentially increased the number of addresses, and eliminated (or postponed) the current address shortage problem.

24 No support for medium-sized organizations The classful A, B, and C octet boundaries were easy to understand and implement, but they did not foster the efficient allocation of a finite address space. Problems resulted from the lack of a network class that was designed to support medium-sized organizations. A /24, which supports 254 hosts, is too small while a /16, which supports 65,534 hosts, is too large. In the past, the Internet has assigned sites with several hundred hosts a single /16 address instead of a couple of /24s addresses. Unfortunately, this has resulted in a premature depletion of the /16 network address space. The only readily available addresses for medium-size organizations are /24s which have the potentially negative impact of increasing the size of the global Internet's routing table.

25

26 IETF Short for Internet Engineering Task Force, the main standards organization for the Internet. The IETF is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual. From Webopedia

27 IANA Short for Internet Assigned Numbers Authority, an organization working under the auspices of the Internet Architecture Board (IAB) that is responsible for assigning new Internet-wide IP addresses. From Webopedia

28 INTERNET PROTOCOL V4 ADDRESS SPACE (last updated 03 August 2004) Originally, all the IPv4 address spaces was managed directly by the IANA. Later parts of the address space were allocated to various other registries to manage for particular purposes or regional areas of the world. RFC 1466 [RFC1466] documents most of these allocations. Block Date Registry - Purpose Notes or Reference /8 Sep 81 IANA - Reserved 001/8 Sep 81 IANA - Reserved 002/8 Sep 81 IANA - Reserved 003/8 May 94 General Electric Company 004/8 Dec 92 Bolt Beranek and Newman Inc. 005/8 Jul 95 IANA - Reserved 006/8 Feb 94 Army Information Systems Center 007/8 Apr 95 IANA - Reserved 008/8 Dec 92 Bolt Beranek and Newman Inc. 009/8 Aug 92 IBM 010/8 Jun 95 IANA - Private Use See [RFC1918] 011/8 May 93 DoD Intel Information Systems 012/8 Jun 95 AT&T Bell Laboratories 013/8 Sep 91 Xerox Corporation 014/8 Jun 91 IANA - Public Data Network 015/8 Jul 94 Hewlett-Packard Company 016/8 Nov 94 Digital Equipment Corporation 017/8 Jul 92 Apple Computer Inc. 018/8 Jan 94 MIT 019/8 May 95 Ford Motor Company 020/8 Oct 94 Computer Sciences Corporation 021/8 Jul 91 DDN-RVN 022/8 May 93 Defense Information Systems Agency 023/8 Jul 95 IANA - Reserved 024/8 May 01 ARIN - Cable Block (Formerly IANA - Jul 95) 025/8 Jan 95 Royal Signals and Radar Establishment 026/8 May 95 Defense Information Systems Agency 027/8 Apr 95 IANA - Reserved 028/8 Jul 92 DSI-North

29 029/8 Jul 91 Defense Information Systems Agency 030/8 Jul 91 Defense Information Systems Agency 031/8 Apr 99 IANA - Reserved 032/8 Jun 94 Norsk Informasjonsteknology 033/8 Jan 91 DLA Systems Automation Center 034/8 Mar 93 Halliburton Company 035/8 Apr 94 MERIT Computer Network 036/8 Jul 00 IANA - Reserved (Formerly Stanford University - Apr 93) 037/8 Apr 95 IANA - Reserved 038/8 Sep 94 Performance Systems International 039/8 Apr 95 IANA - Reserved 040/8 Jun 94 Eli Lily and Company 041/8 May 95 IANA - Reserved 042/8 Jul 95 IANA - Reserved 043/8 Jan 91 Japan Inet 044/8 Jul 92 Amateur Radio Digital Communications 045/8 Jan 95 Interop Show Network 046/8 Dec 92 Bolt Beranek and Newman Inc. 047/8 Jan 91 Bell-Northern Research 048/8 May 95 Prudential Securities Inc. 049/8 May 94 Joint Technical Command (Returned to IANA Mar 98) 050/8 May 94 Joint Technical Command (Returned to IANA Mar 98) 051/8 Aug 94 Deparment of Social Security of UK 052/8 Dec 91 E.I. dupont de Nemours and Co., Inc. 053/8 Oct 93 Cap Debis CCS 054/8 Mar 92 Merck and Co., Inc. 055/8 Apr 95 Boeing Computer Services 056/8 Jun 94 U.S. Postal Service 057/8 May 95 SITA 058/8 Apr 04 APNIC (whois.apnic.net) 059/8 Apr 04 APNIC (whois.apnic.net) 060/8 Apr 03 APNIC (whois.apnic.net) 061/8 Apr 97 APNIC (whois.apnic.net) 062/8 Apr 97 RIPE NCC (whois.ripe.net) 063/8 Apr 97 ARIN (whois.arin.net) 064/8 Jul 99 ARIN (whois.arin.net)

30 065/8 Jul 00 ARIN 066/8 Jul 00 ARIN 067/8 May 01 ARIN 068/8 Jun 01 ARIN 069/8 Aug 02 ARIN 070/8 Jan 04 ARIN 071/8 Aug 04 ARIN 072/8 Aug 04 ARIN 073/8 Sep 81 IANA - Reserved 074/8 Sep 81 IANA - Reserved 075/8 Sep 81 IANA - Reserved 076/8 Sep 81 IANA - Reserved 077/8 Sep 81 IANA - Reserved 078/8 Sep 81 IANA - Reserved 079/8 Sep 81 IANA - Reserved 080/8 Apr 01 RIPE NCC 081/8 Apr 01 RIPE NCC 082/8 Nov 02 RIPE NCC 083/8 Nov 03 RIPE NCC 084/8 Nov 03 RIPE NCC 085/8 Apr 04 RIPE NCC 086/8 Apr 04 RIPE NCC 087/8 Apr 04 RIPE NCC 088/8 Apr 04 RIPE NCC 089/8 Sep 81 IANA - Reserved 090/8 Sep 81 IANA - Reserved 091/8 Sep 81 IANA - Reserved 092/8 Sep 81 IANA - Reserved 093/8 Sep 81 IANA - Reserved 094/8 Sep 81 IANA - Reserved 095/8 Sep 81 IANA - Reserved 096/8 Sep 81 IANA - Reserved 097/8 Sep 81 IANA - Reserved 098/8 Sep 81 IANA - Reserved 099/8 Sep 81 IANA - Reserved 100/8 Sep 81 IANA - Reserved 101/8 Sep 81 IANA - Reserved 102/8 Sep 81 IANA - Reserved 103/8 Sep 81 IANA - Reserved 104/8 Sep 81 IANA - Reserved 105/8 Sep 81 IANA - Reserved 106/8 Sep 81 IANA - Reserved 107/8 Sep 81 IANA - Reserved 108/8 Sep 81 IANA - Reserved 109/8 Sep 81 IANA - Reserved 110/8 Sep 81 IANA - Reserved 111/8 Sep 81 IANA - Reserved 112/8 Sep 81 IANA - Reserved 113/8 Sep 81 IANA - Reserved 114/8 Sep 81 IANA - Reserved 115/8 Sep 81 IANA - Reserved 116/8 Sep 81 IANA - Reserved 117/8 Sep 81 IANA - Reserved 118/8 Sep 81 IANA - Reserved 119/8 Sep 81 IANA - Reserved 120/8 Sep 81 IANA - Reserved 121/8 Sep 81 IANA - Reserved 122/8 Sep 81 IANA - Reserved 123/8 Sep 81 IANA - Reserved 124/8 Sep 81 IANA - Reserved 125/8 Sep 81 IANA - Reserved 126/8 Sep 81 IANA - Reserved 127/8 Sep 81 IANA - Reserved 128/8 May 93 Various Registries 129/8 May 93 Various Registries 130/8 May 93 Various Registries 131/8 May 93 Various Registries 132/8 May 93 Various Registries 133/8 May 93 Various Registries 134/8 May 93 Various Registries 135/8 May 93 Various Registries 136/8 May 93 Various Registries

31 137/8 May 93 Various Registries 138/8 May 93 Various Registries 139/8 May 93 Various Registries 140/8 May 93 Various Registries 141/8 May 93 Various Registries 142/8 May 93 Various Registries 143/8 May 93 Various Registries 144/8 May 93 Various Registries 145/8 May 93 Various Registries 146/8 May 93 Various Registries 147/8 May 93 Various Registries 148/8 May 93 Various Registries 149/8 May 93 Various Registries 150/8 May 93 Various Registries 151/8 May 93 Various Registries 152/8 May 93 Various Registries 153/8 May 93 Various Registries 154/8 May 93 Various Registries 155/8 May 93 Various Registries 156/8 May 93 Various Registries 157/8 May 93 Various Registries 158/8 May 93 Various Registries 159/8 May 93 Various Registries 160/8 May 93 Various Registries 161/8 May 93 Various Registries 162/8 May 93 Various Registries 163/8 May 93 Various Registries 164/8 May 93 Various Registries 165/8 May 93 Various Registries 166/8 May 93 Various Registries 167/8 May 93 Various Registries 168/8 May 93 Various Registries 169/8 May 93 Various Registries 170/8 May 93 Various Registries 171/8 May 93 Various Registries 172/8 May 93 Various Registries 173/8 Apr 03 IANA - Reserved 174/8 Apr 03 IANA - Reserved 175/8 Apr 03 IANA - Reserved 176/8 Apr 03 IANA - Reserved 177/8 Apr 03 IANA - Reserved 178/8 Apr 03 IANA - Reserved 179/8 Apr 03 IANA - Reserved 180/8 Apr 03 IANA - Reserved 181/8 Apr 03 IANA - Reserved 182/8 Apr 03 IANA - Reserved 183/8 Apr 03 IANA - Reserved 184/8 Apr 03 IANA - Reserved 185/8 Apr 03 IANA - Reserved 186/8 Apr 03 IANA - Reserved 187/8 Apr 03 IANA - Reserved 188/8 May 93 Various Registries 189/8 Apr 03 IANA - Reserved 190/8 Apr 03 IANA - Reserved 191/8 May 93 Various Registries 192/8 May 93 Various Registries 193/8 May 93 RIPE NCC 194/8 May 93 RIPE NCC 195/8 May 93 RIPE NCC 196/8 May 93 Various Registries 197/8 May 93 IANA - Reserved 198/8 May 93 Various Registries 199/8 May 93 ARIN 200/8 Nov 02 LACNIC 201/8 Apr 03 LACNIC 202/8 May 93 APNIC 203/8 May 93 APNIC 204/8 Mar 94 ARIN 205/8 Mar 94 ARIN 206/8 Apr 95 ARIN 207/8 Nov 95 ARIN 208/8 Apr 96 ARIN 209/8 Jun 96 ARIN 210/8 Jun 96 APNIC

32 211/8 Jun 96 APNIC 212/8 Oct 97 RIPE NCC 213/8 Mar 99 RIPE NCC 214/8 Mar 98 US-DOD 215/8 Mar 98 US-DOD 216/8 Apr 98 ARIN 217/8 Jun 00 RIPE NCC 218/8 Dec 00 APNIC 219/8 Sep 01 APNIC 220/8 Dec 01 APNIC 221/8 Jul 02 APNIC 222/8 Feb 03 APNIC 223/8 Apr 03 IANA - Reserved 224/8 Sep 81 IANA - Multicast 225/8 Sep 81 IANA - Multicast 226/8 Sep 81 IANA - Multicast 227/8 Sep 81 IANA - Multicast 228/8 Sep 81 IANA - Multicast 229/8 Sep 81 IANA - Multicast 230/8 Sep 81 IANA - Multicast 231/8 Sep 81 IANA - Multicast 232/8 Sep 81 IANA - Multicast 233/8 Sep 81 IANA - Multicast 234/8 Sep 81 IANA - Multicast 235/8 Sep 81 IANA - Multicast 236/8 Sep 81 IANA - Multicast 237/8 Sep 81 IANA - Multicast 238/8 Sep 81 IANA - Multicast 239/8 Sep 81 IANA - Multicast 240/8 Sep 81 IANA - Reserved 241/8 Sep 81 IANA - Reserved 242/8 Sep 81 IANA - Reserved 243/8 Sep 81 IANA - Reserved 244/8 Sep 81 IANA - Reserved 245/8 Sep 81 IANA - Reserved 246/8 Sep 81 IANA - Reserved 247/8 Sep 81 IANA - Reserved 248/8 Sep 81 IANA - Reserved 249/8 Sep 81 IANA - Reserved 250/8 Sep 81 IANA - Reserved 251/8 Sep 81 IANA - Reserved 252/8 Sep 81 IANA - Reserved 253/8 Sep 81 IANA - Reserved 254/8 Sep 81 IANA - Reserved 255/8 Sep 81 IANA - Reserved

33 Summary Table for Specialized Address Blocks Address Block Present Use Reference /8 "This" Network [RFC1700, page 4] /8 Private-Use Networks [RFC1918] /8 Public-Data Networks [RFC1700, page 181] /8 Cable Television Networks /8 Reserved but subject to allocation [RFC1797] /8 Loopback [RFC1700, page 5] /16 Reserved but subject to allocation /16 Link Local /12 Private-Use Networks [RFC1918] /16 Reserved but subject to allocation /24 Reserved but subject to allocation /24 Test-Net /24 6to4 Relay Anycast [RFC3068] /16 Private-Use Networks [RFC1918] /15 Network Interconnect Device Benchmark Testing [RFC2544] /24 Reserved but subject to allocation /4 Multicast [RFC3171] /4 Reserved for Future Use [RFC1700, page 4]

34 The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets: (10/8 prefix) (172.16/12 prefix) ( /16 prefix) We will refer to the first block as "24-bit block", the second as "20-bit block", and to the third as "16-bit" block. Note that (in pre-cidr notation) the first block is nothing but a single class A network number, the second is a set of 16 contiguous class B network numbers, and the third is a set of 256 contiguous class C network numbers.

35 Total address space: chunks or "/8's", each of which spans 16,777,216 address values. IPv4 Address Space The blocks of addresses - from to reserved for Multicast use. - from to reserved for future definition. The address blocks /8, /8, /8 are reserved, as are the address ranges used for private networks and other reserved uses. See RFC The remaining addresses, the equivalent of /8 address blocks form the pool of unicast addresses which are used for the Internet. Unicast /8s 85.91% Multicast /8s 6.25% IETF Res /8s 7.84%

36 IANA allocations nowdays Allocated /8s 55.83% IANA Pool /8s 30.08% Multicast /8s 6.25% IETF Res /8s 7.84%

37 IPv4 IANA Projections The post-1995 data has been fitted to an exponential growth model (a model that assumes growth is proportional to the total size of the network) The extrapolation of this model to the point of address pool exhaustion is shown here.

38 The END of IPv4 IPv4 Address Space Exhaustion Predictors: Application of best fit models to historical data relating to the growth in the address space advertised in the BGP routing table. The underlying assumptions made in this predictive model is that the previous drivers in address consumption will continue to determine future consumption rates, and that growth in consumption rates will continue to operate in a fashion where the growth rate is constant rather than increasing or decreasing. Source: Prediction updated: 23 October 2005 (now) Exhaustion of the IPv4 Unallocated Address Pool March 2013 Complete Exhaustion of all available IPv4 Address Space: August 2022!!!

39 Summary of problems - Address space depletion - Bloating of Internet routing tables. - Bourocratic loads: local administrators had to request another network number from the Internet before a new network could be installed at their site. The subsequent history of Internet addressing is focused on a series of steps that overcome these addressing issues and have supported the growth of the global Internet.

40 Subnetting

41 Subnetting One solution is to allow a network to be split in several parts for internal use, but still act as a single network to the outside world.

42 Example A campus network Here each of the ethernets has his own router connected to the main router

43 How does it work When a packet comes into the main router, how does this know which subnet (Ethernet) to give it to? Having a host table with 65K entries each with the responsable router is impractical A better way is that of devoting a part of the host address to the specification of the router address

44 Fixed Length Mask Subnetting In practice some bits are taken away from the host number to create a subnet number This adds another level of hierarchy to the IP addressing structure. Instead of the classful two-level hierarchy, subnetting supports a threelevel hierarchy.

45 Subnet Mask To implement subnetting the main router needs a subnet mask that indicates the split between the network+subnetwork number and host: the subnet mask tells the net router where the host addresses starts. The bits of the subnet mask are set to 1 if the system examining the address should treat the corresponding bit in the IP address as part of the extended-network- prefix. The bits in the mask are set to 0 if the system should treat the bit as part of the host-number.

46 Extended-Network-Prefix Length The standards describing modern routing protocols often refer to the extended-network-prefix- length rather than the subnet mask. The prefix length is equal to the number of contiguous one-bits in the traditional subnet mask. However, it is important to note that modern routing protocols still carry the subnet mask. There are no Internet standard routing protocols that have a one-byte field in their header that contains the number of bits in the extended-network prefix. Rather, each routing protocol is still required to carry the complete four-octet subnet mask.

47 How does it work? Address: Subnet Mask: AND Network ID: In order to route an incoming packet the main router uses the mask by performing a logical AND operation, so as to extract the network address from the overall address, and hands the packet to the corresponding router. In the last column of the above example we have a class C address with a mask of length 26 which tells us that the host portion of the address must be split into the subnet prefix 10 and the host address

48 How it works without subnetting Each router has a table listing some number of (network, 0) IP addresses and some number of (this-network, host) IP addresses: associated with each table is the network interface to use to reach the destination.the first table is for distant networks, the second for local hosts. When an IP packet arrives its destination address is looked up in the routing table: if it is for a distant network it is forwarded to the router indicated in the table; if it is for a local host (e.g. on the touter LAN) it is sent directly to dht destination.

49 How it works with subnetting When subnetting is introduced the routing tables are changed, adding entries of the form (this-network, subnet, 0) and (this-network, this-subnet, host) The first is used to reach other subnets, the second to reach the hosts of the local subnet. Notice that in this way the router does not have to know the details about the hosts on other subnets: the router will - take the IP address - perform an AND with the subnet mask getting rid of the host number - look up the resulting subnet number in the routing table.

50 Benefits The size of the global Internet routing table does not grow because the site administrator does not need to obtain additional address space and the routing advertisements for all of the subnets are combined into a single routing table entry. The local administrator has the flexibility to deploy additional subnets without obtaining a new network number from the Internet. Route flapping (i.e., the rapid changing of routes) within the private network does not affect the Internet routing table since Internet routers do not know about the reachability of the individual subnets - they just know about the reachability of the parent network number.

51 Subnet Design Considerations The deployment of an addressing plan requires careful thought on the part of the network administrator. There are four key questions that must be answered before any design should be undertaken: 1) How many total subnets does the organization need today? 2) How many total subnets will the organization need in the future? 3) How many hosts are there on the organization's largest subnet today? 4) How many hosts will there be on the organization's largest subnet in the future?

52 All Zero and all one hosts Recall that according to Internet practices, the host-number field of an IP address cannot contain all 0-bits or all 1-bits: - the all-0s host-number identifies the base network (or subnetwork) number, -the all-1s host-number represents the broadcast address for the network (or subnetwork). In practice with n bits one will be able to address 2^n-2 hosts

53 How to subnet a network To subnet a network, extend the natural mask using some of the bits from the host ID portion of the address to create a subnetwork ID. For example, given a Class C network of which has a natural mask of , you can create subnets in this manner: sub ---- By extending the mask to be , you have taken three bits (indicated by "sub") from the original host portion of the address and used them to make subnets. With these three bits, it is possible to create eight subnets. With the remaining five host ID bits, each subnet can have up to 32 host addresses, 30 of which can actually be assigned to a device since host ids of all zeros or all ones are not allowed. So, with this in mind, these subnets have been created host address range 1 to host address range 33 to host address range 65 to host address range 97 to host address range 129 to host address range 161 to host address range 193 to host address range 225 to 254

54 Example Subnetting a class C network Three bits are reserved for the subnet addresses Five bits are reserved for the host addresses This means that there is going to be room for 2^3 = 8 subnets each with at most 2^5-2 = 30 hosts

55 More subnets => less hosts This brings up an interesting point. The more host bits you use for a subnet mask, the more subnets you have available. However, the more subnets available, the less host addresses available per subnet. For example, a Class C network of and a mask of (/27) allows you to have eight subnets, each with 32 host addresses (30 of which could be assigned to devices). If you use a mask of (/28), the break down is: sub --- Since you now have four bits to make subnets with, you only have four bits left for host addresses. So in this case you can have up to 16 subnets, each of which can have up to 16 host addresses (14 of which can be assigned to devices).

56 Class C Host/Subnet Table Class C Subnet Effective Effective Number of Subnet Bits Mask Subnets Hosts Mask Bits / / / / / / * /31 Notice that an exception to the 2^n-2 rule is 31-bit prefixes, marked with an asterisk ( * ).

57 Subnetting a Class B network Take a look at how a Class B network might be subnetted. If you have network ,then you know that its natural mask is or /16. Extending the mask to anything beyond means you are subnetting. You can quickly see that you have the ability to create a lot more subnets than with the Class C network. If you use a mask of (/21), how many subnets and hosts per subnet does this allow for? sub You are using five bits from the original host bits for subnets. This will allow you to have 32 subnets (25). After using the five bits for subnetting, you are left with 11 bits for host addresses. This will allow each subnet so have 2048 host addresses (211), 2046 of which could be assigned to devices.

58 Example Subnetting a class B network Nine bits are reserved for the subnet addresses Seven bits are reserved for the host addresses This means that there is going to be room for 2^9 = 512 subnets each with at most 2^7-2 = 126 hosts

59 Class B Host/Subnet Table Class B Subnet Effective Effective Number of Subnet Bits Mask Subnets Hosts Mask Bits / / / / / / / / / / / / / / * /31

60 Class A Host/Subnet Table Class A Number of Bits Borrowed Subnet Effective Number of Number of Subnet from Host Portion Mask Subnets Hosts/Subnet Mask Bits / / / / / / / / / / / / / / / / / / / / / / * /31

61 Subnetting Example The first entry in the Class A table (/10 subnet mask) borrows two bits (the leftmost bits) from the host portion of the network for subnetting, then with two bits you have four (2 2 ) combinations, 00, 01, 10, and 11. Each of these will represent a subnet. Binary Notation Decimal Notation xxxx xxxx / > X.0.0.0/10 xxxx xxxx / > X /10 xxxx xxxx / > X /10 xxxx xxxx / > X /10 Note: The subnet zero and all-ones subnet are included in the effective number of subnets as shown in the third column.

62 Time to work up Refer to the file Sample exercises.pdf to see a few worked out examples of fixed mask subnetting.

63 Variable Length Subnet Masks (VLSM) In 1987, RFC 1009 specified that a subnetted network could use more than one subnet mask. When an IP network is assigned more than one subnet mask, it is considered a network with variable length subnet masks..

64 VLSM Benefits Efficient use of the organization s assigned IP address space. Route aggregation.

65 Efficient Use of the Organization's Assigned IP Address Space VLSM supports more efficient use of an organization's assigned IP address space. One of the major problems with the earlier limitation of supporting only a single subnet mask across a given network-prefix was that once the mask was selected, it locked the organization into a fixed-number of fixed-sized subnets. For example, assume that a network administrator decided to configure the /16 network with a /22 extended-network-prefix.

66 ...the waste... A /22 extended-network prefix permits 64 subnets (2^6 ), each of which supports a maximum of 1,022 hosts (2^10-2). This is fine if the organization wants to deploy a number of large subnets, but what about the occasional small subnet containing only 20 or 30 hosts? Since a subnetted network could have only a single mask, the network administrator was still required to assign the 20 or 30 hosts to a subnet with a 22-bit prefix. This assignment would waste approximately 1,000 IP host addresses for each small subnet deployed!

67 avoided. One solution to this problem was to allow a subnetted network to be assigned more than one subnet mask. Assume that the network administrator is also allowed to configure the /16 network with a /26 extended-network-prefix. A /16 network address with a /26 extended-network prefix permits 1024 subnets (2^10 ), each of which supports a maximum of 62 hosts (2^6-2). The /26 prefix would be ideal for small subnets with less than 60 hosts, while the /22 prefix is well suited for larger subnets containing up to 1000 hosts.

68 VLSM. Route aggregation VLSM allows the recursive division of an organization s address space. It can be aggregated to reduce the amount of routing information at the top level.

69 Route Aggregation VLSM also allows the recursive division of an organization's address space so that it can be reassembled and aggregated to reduce the amount of routing information at the top level. This allows the detailed structure of routing information for one subnet group to be hidden from routers in another subnet group. Conceptually, a network is first divided into subnets, some of the subnets are further divided into sub-subnets, and so on. The /8 network is first configured with a /16 extended-network-prefix. The /16 subnet is then configured with a /24 extended-network-prefix and the /16 subnet is configured with a /19 extended-network-prefix.

70 Reducing Routing Table Size A planned and thoughtful allocation of VLSM can reduce the size of an organization's routing tables. Notice how Router D is able to summarize the six subnets behind it into a single advertisement ( /24) and how Router B is able to aggregate all of subnets behind it into a single advertisement. Likewise, Router C is able to summarize the six subnets behind it into a single advertisement ( /16). Finally, since the subnet structure is not visible outside of the organization, Router A injects a single route into the global Internet's routing table / 8 (or 11/8).

71 VLSM operation Conceptually, a network is divided into subnets, some of the subnets are further divided into sub-subnets, and some of the sub-subnets are divided into sub 2 -subnets.

72 VLSM permits the recursive division of a netrwork prefix / / / / / / / / / / / / / / / / / / /19

73 VLSM operation The recursive process does not require the same extended-network-prefix be assigned at each level of recursion. The recursive subdivision can be carried out as far as the network administrator needs to take it.

74 VLSM Design Considerations At each level of the hierarchy: 1) How many total subnets does this level need today? 2) How many total subnets does this level need in the future? 3) How many hosts are there on this level s largest subnet today? 4) How many hosts will there be on this level s largest subnet in the future?

75 VLSM Design Considerations (example) Assume a network is spread out over a number of sites. An organization has 3 campuses today. It will need 3 bits of subnetting to allow growth (8 subnets). Within each campus a second level of subnetting will identify a building. Within each building a third level of subnetting will identify an individual workgroup.

76 VLSM Design Considerations (example) From this hierarchical model, the top level is determined by the number of campuses. The mid-level by the number of buildings at each site. The lowest level by the number of workgroups.

77 VLSM Design Considerations (example) The deployment of a hierarchical subnetting scheme requires careful planning. At the bottom level, the designer must be sure that the leaf subnets are large enough to support the required number of hosts. The addresses from each site will be aggregable into a single address block that keeps the backbone routing tables from becoming too large.

78 Requierments for VLSM Deployment Three prerequisites: The routing protocols must carry extendednetwork-prefix information with each routing update. All routers must implement a consistent forwarding algorithm based on the longest match. For route aggregation to occur, addresses must be assigned so that they have topological significance.

79 Requierments for VLSM Deployment Routing protocols OSPF, IS-IS, RIP-2, EIGRP allow the deployment of VLSM by providing the extended-network-prefix length or mask value along with each route advertisement. This permits each subnetwork to be advertised with its corresponding prefix length or mask.

80 Requirements for VLSM Deployment Forwarding algorithm based on longest match A route with a longer e-n-p describes a smaller set of destinations than the same route with a shorter e-n-p. Then, a route with a longer e-n-p is said to be more specific. A route with a shorter e-n-p is said to be less specific. Routers must use the route with the longest matching e-n-p (most specific matching route) when forwarding traffic.

81 Requierments for VLSM Deployment Example If a packet destination IP address is and there are 3 network prefixes in the routing table ( /24, /16, and /8), the router would select the route to /24 because it has the longest match with the destination IP address.

82 Requirements for the Deployment of VLSM The successful deployment of VLSM has three prerequisites: The routing protocols must carry extended-network-prefix information with each route advertisement. All routers must implement a consistent forwarding algorithm based on the "longest match." For route aggregation to occur, addresses must be assigned so that they have topological significance.

83 Requierments for VLSM Deployment Topological significant address assignment Hierarchical routing requires that addresses be assigned to reflect the actual network topology. Routing information is reduced by taking the set of addresses assigned to a particular region of the topology, and aggregating them into a single routing update for the entire set. This can be done recursively at various points within the hierarchy of the routing topology.

84 Requierments for VLSM Deployment Topological significant address assignment If addresses do not have a topological significance, aggregation cannot be performed and the size of routing tables would not be reduced.

85 Time to work up Refer to the files Sample exercises.pdf VLSM_01.pdf VLSM_02.pdf to se a few worked out examples of Variable Lenght Subnet Masking.

86 Supernetting: Classless Inter-Domain Routing (CIDR) CIDR was officially documented in September 1993 in RFC 1517, 1518, 1519, and CIDR supports two important features that benefit the global Internet routing system: - CIDR eliminates the traditional concept of Class A, Class B, and Class C network addresses. This enables the efficient allocation of the IPv4 address space which will allow the continued growth of the Internet until IPv6 is deployed. - CIDR supports route aggregation where a single routing table entry can represent the address space of perhaps thousands of traditional classful routes. This allows a single routing table entry to specify how to route traffic to many individual network addresses. Route aggregation helps control the amount of routing information in the Internet's backbone routers, reduces route flapping (rapid changes in route availability), and eases the local administrative burden of updating external routing information.

87 CIDR and VLSM CIDR and VLSM are essentially the same thing since they both allow a portion of the IP address space to be recursively divided into subsequently smaller pieces. The difference is that with VLSM, the recursion is performed on the address space previously assigned to an organization and is invisible to the global Internet. CIDR, on the other hand, permits the recursive allocation of an address block by an Internet Registry to a high-level ISP, to a mid-level ISP, to a low-level ISP, and finally to a private organization's network.

88 /20 Bitwise Contiguous Address Blocks In a classless environment, prefixes are viewed as bitwise contiguous blocks of the IP address space. For example, all prefixes with a /20 prefix represent the same amount of address space (2 12 or 4,096 host addresses): a /20 prefix can be assigned to a traditional Class A, Class B, or Class C network number. See the following /20 blocks represent 4,096 host addresses /20, /20, and /20.

89 This Table provides information about the most commonly deployed CIDR address blocks. CIDR Address Blocks

90 CIDR Reduces the Size of Internet Routing Tables

91 Organization A Changes Network Providers

92 IP Version 6 (IPv6): Expanded Addressing IPv4 uses 32-bit addresses, which potentially can address up to 2 32 nodes. However, the combination of network and local address hierarchy and reserved address space for special handling such as loopback and broadcast reduces the number of addressable nodes. At the same time, the exponential growth of computer networks in recent years indicates the outgrowth of addressable node using 32-bit addresses. The IPv6 address size has been increased to 128 bits. In addition to increased address size, IPv6 eliminated broadcast address and added the notion of anycast address, which can be used to send a packet to any one of a group of nodes.

93 References A good reference is the following (it contains pointers to subnet calculators):