A few notes. The following slides are NOT from the online curriculum. However, they do cover the same topics using different examples.

Transcription

1 Objectives This module explores the evolution and extension of IPv4, including the key scalability features that engineers have added to it over the years: Subnetting Classless interdomain routing (CIDR) Variable length subnet masking (VLSM) Route summarization Finally, this module examines advanced IP implementation techniques such as the following: IP unnumbered Dynamic Host Configuration Protocol (DHCP) Helper addresses 1

2 A few notes The following slides are NOT from the online curriculum. However, they do cover the same topics using different examples. 2

3 IPv4 Address Classes 3

4 IPv4 Address Classes No medium size host networks In the early days of the Internet, IP addresses were allocated to organizations based on request rather than actual need. 4

5 IPv4 Address Classes Class D Addresses A Class D address begins with binary 1110 in the first octet. First octet range 224 to 239. Class D address can be used to represent a group of hosts called a host group, or multicast group. Class E Addresses First octet of an IP address begins with 1111 Class E addresses are reserved for experimental purposes and should not be used for addressing hosts or multicast groups. 5

6 IP addressing crisis Address Depletion Internet Routing Table Explosion 6

7 IPv4 Addressing Subnet Mask One solution to the IP address shortage was thought to be the subnet mask. Formalized in 1985 (RFC 950), the subnet mask breaks a single class A, B or C network in to smaller pieces. 7

8 Subnet Example Given the Class B address Class B Using /24 subnet... Network Network Host Host Network Network Subnet Host Internet routers still see this net as But internal routers think all these addresses are on different networks, called subnetworks 8

9 Subnet Example Network Network Subnet Host Using the 3rd octet, was divided into: and so on... 9

10 All Zeros and All Ones Subnets Using the All Ones Subnet There is no command to enable or disable the use of the all-ones subnet, it is enabled by default. Router(config)#ip subnet-zero The use of the all-ones subnet has always been explicitly allowed and the use of subnet zero is explicitly allowed since Cisco IOS version RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is obsolete! Modern software will be able to utilize all definable networks." Today, the use of subnet zero and the all-ones subnet is generally accepted and most vendors support their use, though, on certain networks, particularly the ones using legacy software, the use of subnet zero and the all-ones subnet can lead to problems. CCO: Subnet Zero and the All-Ones Subnet 8.shtml 10

11 Long Term Solution: IPv6 IP v6, or IPng (IP the Next Generation) uses a 128-bit address space, yielding 340,282,366,920,938,463,463,374,607,431,768,211,456 possible addresses. IPv6 has been slow to arrive IPv4 revitalized by new features, making IPv6 a luxury, and not a desperately needed fix IPv6 requires new software; IT staffs must be retrained IPv6 will most likely coexist with IPv4 for years to come. Some experts believe IPv4 will remain for more than 10 years. 11

12 Short Term Solutions: IPv4 Enhancements CIDR (Classless Inter-Domain Routing) RFCs 1517, 1518, 1519, 1520 VLSM (Variable Length Subnet Mask) RFC 1009 Private Addressing - RFC 1918 NAT/PAT (Network Address Translation / Port Address Translation) RFC 12

13 CIDR (Classless Inter-Domain Routing) By 1992, members of the IETF were having serious concerns about the exponential growth of the Internet and the scalability of Internet routing tables. The IETF was also concerned with the eventual exhaustion of 32-bit IPv4 address space. Projections were that this problem would reach its critical state by 1994 or IETF s response was the concept of Supernetting or CIDR, cider. To CIDR-compliant routers, address class is meaningless. The network portion of the address is determined by the network subnet mask, network-prefix or prefix-length (/8, /19, etc.) The network address is NOT determined by the first octet (first two bits), /16 or /19 CIDR helped reduced the Internet routing table explosion with supernetting and reallocation of IPv4 address space. 13

14 Active BGP entries Report last updated at Thu, 16 Jan

15 CIDR (Classless Inter-Domain Routing) First deployed in 1994, CIDR dramatically improves IPv4 s scalability and efficiency by providing the following: Eliminates traditional Class A, B, C addresses allowing for more efficient allocation of IPv4 address space. Supporting route aggregation (summarization), also known as supernetting, where thousands of routes could be represented by a single route in the routing table. Route aggregation also helps prevent route flapping on Internet routers using BGP. Flapping routes can be a serious concern with Internet core routers. CIDR allows routers to aggregate, or summarize, routing information and thus shrink the size of their routing tables. Just one address and mask combination can represent the routes to multiple networks. Used by IGP routers within an AS and EGP routers between AS. 15

16 a router must maintain individual routing table entries for these class B networks. With CIDR, a router can summarize these routes into eight networks by using a 13-bit prefix: /13 Steps: 1. Count the number of left-most matching bits, /13 2. Add all zeros after the last matching bit: =

17 CIDR (Classless Inter-Domain Routing) By using a prefix address to summarizes routes, administrators can keep routing table entries manageable, which means the following More efficient routing A reduced number of CPU cycles when recalculating a routing table, or when sorting through the routing table entries to find a match Reduced router memory requirements Route summarization is also known as: Route aggregation Supernetting Supernetting is essentially the inverse of subnetting. CIDR moves the responsibility of allocation addresses away from a centralized authority (InterNIC). Instead, ISPs can be assigned blocks of address space, which they can then parcel out to customers. 17

18 ISP/NAP Hierarchy - The Internet: Still hierarchical after all these years. Jeff Doyle (Tries to be anyways!) NAP (Network Access Point) Network Service Provider Network Service Provider Regional Service Provider Regional Service Provider Regional Service Provider Regional Service Provider ISP ISP ISP ISP ISP ISP ISP ISP Subscribers Subscribers Subscribers Subscribers Subscribers Subscribers Subscribers Subscribers 18

19 Supernetting Example Company XYZ needs to address 400 hosts. Its ISP gives them two contiguous Class C addresses: / /24 Company XYZ can use a prefix of /23 to supernet these two contiguous networks. (Yielding 510 hosts) / / /24 23 bits in common 19

20 Supernetting Example With the ISP acting as the addressing authority for a CIDR block of addresses, the ISP s customer networks, which include XYZ, can be advertised among Internet routers as a single supernet. 20

21 CIDR and the Provider Another example of route aggregation. 21

22 CIDR and the provider / /23 Even Better: / / / / Summarization from the customer networks to their provider. (As long as there are no other routes elsewhere within this range, well ) / / /

23 /25 CIDR and the provider /23 Further summarization happens with the next upstream provider / / / / bits in common 23

24 CIDR Restrictions Dynamic routing protocols must send network address and mask (prefix-length) information in their routing updates. In other words, CIDR requires classless routing protocols for dynamic routing. However, you can still configure summarized static routes, after all, that is what a /0 route is. 24

25 Summarized and Specific Routes: Longest-bit Match (more later) Merida Summarized Update Specific Route Update / / /24 Quito Cartago / / /24 Merida receives a summarized /16 update from Quito and a more specific /24 update from Cartago. Merida will include both routes in the routing table. Merida will forward all packets matching at least the first 24 bits of to Cartago (172/16/5/0/24), longest-bit match. Merida will forward all other packets matching at least the first 16 bits to Quito ( /16). 25

26 Short Term Solutions: IPv4 Enhancements CIDR (Classless Inter-Domain Routing) RFCs 1517, 1518, 1519, 1520 VLSM (Variable Length Subnet Mask) RFC 1009 Private Addressing - RFC 1918 NAT/PAT (Network Address Translation / Port Address Translation) RFC 26

27 VLSM (Variable Length Subnet Mask) Limitation of using only a single subnet mask across a given network-prefix (network address, the number of bits in the mask) was that an organization is locked into a fixed-number of of fixed-sized subnets. 1987, RFC 1009 specified how a subnetted network could use more than one subnet mask. VLSM = Subnetting a Subnet If you know how to subnet, you can do VLSM! 27

28 VLSM Simple Example / /16 1st octet 2nd octet 3rd octet 4th octet 10 Host Host Host 10 Subnet Host Host / Host Host / Host Host / Host Host 10.n.0.0/16 10 Host Host / Host Host Subnetting a /8 subnet using a /16 mask gives us 256 subnets with 65,536 hosts per subnet. Let s take the /16 subnet and subnet it further 28

29 VLSM Simple Example Note: /16 is now a summary of all of the /24 subnets. Summarization coming soon! Network Subnet Host Host / Host Host / Subnet Host / Host / Host 10.2.n.0/ Host / Host 29

30 VLSM Simple Example /8 subnetted using /16 Subnet 1 st host Last host Broadcast / / /16 sub-subnetted using /24 Subnet 1 st host Last host Broadcast / / / Etc / / Etc /

31 VLSM Simple Example Subnets / / / / / /24 Etc / /16 Etc /16 An example of VLSM, NOT of good network design / / / / / / / / / / /16 Your network can now have 255 /16 subnets with 65,534 hosts each AND 256 /24 subnets with 254 hosts each. All you need to make it work is a classless routing protocol that passes the subnet mask with the network address in the routing updates. Classless routing protocols: RIPv2, EIGRP, OSPF, IS-IS, BGPv4 (coming) / / /24 31

32 Another VLSM Example using /30 subnets /24 network subnetted into eight /27 ( ) subnets /27 subnet, subnetted into eight /30 ( ) subnets This network has seven /27 subnets with 30 hosts each AND eight /30 subnets with 2 hosts each. /30 subnets are very useful for serial networks. 32

33 / /30 Hosts Bcast 2 Hosts / & / & / & / & / & / & / & / &

34 / / / / / / / / / / / / / /27 This network has seven /27 subnets with 30 hosts each AND seven /30 subnets with 2 hosts each (one left over). /30 subnets with 2 hosts per subnet do not waste host addresses on serial networks. 34

35 VLSM and the Routing Table Routing Table without VLSM RouterX#show ip route /27 is subnetted, 4 subnets C is directly connected, Serial0 C is directly connected, Serial1 C is directly connected, Serial2 C is directly connected, FastEthernet0 Routing Table with VLSM RouterX#show ip route C C C C Displays one subnet mask for all child routes. Classful mask is assumed for the parent route /24 is variably subnetted, 4 subnets, 2 masks /30 is directly connected, Serial /30 is directly connected, Serial /30 is directly connected, Serial /27 is directly connected, FastEthernet0 Parent Route shows classful mask instead of subnet mask of the child routes. Each Child Routes includes its subnet mask. Each child routes displays its own subnet mask. Classful mask is included for the parent route. 35

36 Final Notes on VLSM Whenever possible it is best to group contiguous routes together so they can be summarized (aggregated) by upstream routers. (coming soon!) Even if not all of the contiguous routes are together, routing tables use the longest-bit match which allows the router to choose the more specific route over a summarized route. Coming soon! You can keep on sub-subnetting as many times and as deep as you want to go. You can have various sizes of subnets with VLSM. 36

37 Route flapping Route flapping occurs when a router interface alternates rapidly between the up and down states. Route flapping, and it can cripple a router with excessive updates and recalculations. However, the summarization configuration prevents the RTC route flapping from affecting any other routers. The loss of one network does not invalidate the route to the supernet. While RTC may be kept busy dealing with its own route flap, RTZ, and all upstream routers, are unaware of any downstream problem. Summarization effectively insulates the other routers from the problem of route flapping. 37

38 Short Term Solutions: IPv4 Enhancements CIDR (Classless Inter-Domain Routing) RFCs 1517, 1518, 1519, 1520 VLSM (Variable Length Subnet Mask) RFC 1009 Private Addressing - RFC 1918 NAT/PAT (Network Address Translation / Port Address Translation) RFC 38

39 Private IP addresses (RFC 1918) If addressing any of the following, these private addresses can be used instead of globally unique addresses: A non-public intranet A test lab A home network Global addresses must be obtained from a provider or a registry at some expense. 39

40 Discontiguous subnets Mixing private addresses with globally unique addresses can create discontiguous subnets. Not the main cause however Discontiguous subnets, are subnets from the same major network that are separated by a completely different major network or subnet. Question: If a classful routing protocol like RIPv1 or IGRP is being used, what do the routing updates look like between Site A router and Site B router? 40

41 Discontiguous subnets Classful routing protocols, notably RIPv1 and IGRP, can t support discontiguous subnets, because the subnet mask is not included in routing updates. RIPv1 and IGRP automatically summarize on classful boundaries. Site A and Site B are all sending each other the classful address of /24. A classless routing protocol (RIPv2, EIGRP, OSPF) would be needed: to not summarize the classful network address and to include the subnet mask in the routing updates. 41

42 Discontiguous subnets RIPv2 and EIGRP automatically summarize on classful boundaries. When using RIPv2 and EIGRP, to disable automatic summarization (on both routers): Router(config-router)#no auto-summary SiteA now receives /27 SiteB now receives /27 42

43 Short Term Solutions: IPv4 Enhancements CIDR (Classless Inter-Domain Routing) RFCs 1517, 1518, 1519, 1520 VLSM (Variable Length Subnet Mask) RFC 1009 Private Addressing - RFC 1918 NAT/PAT (Network Address Translation / Port Address Translation) RFC 43

44 Network Address Translation (NAT) NAT: Network Address Translatation NAT, as defined by RFC 1631, is the process of swapping one address for another in the IP packet header. In practice, NAT is used to allow hosts that are privately addressed to access the Internet. 44

45 Network Address Translation (NAT) TCP Source Port 1026 TCP Source Port 1026 NAT translations can occur dynamically or statically. The most powerful feature of NAT routers is their capability to use port address translation (PAT), which allows multiple inside addresses to map to the same global address. This is sometimes called a many-to-one NAT TCP Source Port TCP Source Port 1924 With PAT, or address overloading, literally hundreds of privately addressed nodes can access the Internet using only one global address. The NAT router keeps track of the different conversations by mapping TCP and UDP port numbers. 45

46 Using IP unnumbered There are certain drawbacks that come with using IP unnumbered: The use of ping cannot determine whether the interface is up because the interface has no IP address. A network IOS image cannot boot over an unnumbered serial interface. IP security options cannot be supported on an unnumbered interface. 46

47 DHCP DHCP overview DHCP operation Configuring IOS DHCP server Easy IP 47

48 DHCP overview Administrators set up DHCP servers to assign addresses from predefined pools. DHCP servers can also offer other information: DNS server addresses WINS server addresses Domain names Most DHCP servers also allow the ability to define specifically what client MAC addresses can be serviced and to automatically assign the same number to a particular host each time. Note: BootP was originally defined in RFC 951 in It is the predecessor of DHCP, and it shares some operational characteristics. Both protocols use UDP ports 67 and 68, which are well known as BootP ports because BootP came before DHCP. 48

49 DHCP operation The client sends a DHCPREQUEST broadcast to all nodes. If the client finds the offer agreeable, it will send another broadcast. This broadcast is a DHCPREQUEST, specifically requesting those particular IP parameters. Why does the client broadcast the request instead of unicasting it to the server? A broadcast is used because the very first message, the DHCPDISCOVER, may have reached more than one DHCP server. After all, it was a broadcast. If more than one server makes an offer, the broadcasted DHCPREQUEST lets the servers know which offer was accepted, which is usually the first offer received. 49

50 Configuring IOS DHCP server Basic More options Note: The network statement enables DHCP on any router interfaces belonging to that network. 50

51 Configuring IOS DHCP server (1 of 2) 51

52 Configuring IOS DHCP server (2 of 2) 52

53 Easy IP 53

54 Using helper addresses 54

55 Configuring IP helper addresses By default, the ip helper-address command forwards the eight UDPs services. 55

56 Configuring IP helper addresses Broadcast Unicast To configure RTA e0, the interface that receives the Host A broadcasts, to relay DHCP broadcasts as a unicast to the DHCP server, use the following commands: RTA(config)#interface e0 RTA(config-if)#ip helper-address

57 Configuring IP helper addresses Broadcast Unicast Helper address configuration that relays broadcasts to all servers on the segment. RTA(config)#interface e0 RTA(config-if)#ip helper-address But will RTA forward the broadcast? 57

58 Directed Broadcast Notice that the RTA interface e3, which connects to the server farm, is not configured with helper addresses. However, the output shows that for this interface, directed broadcast forwarding is disabled. This means that the router will not convert the logical broadcast into a physical broadcast with a Layer 2 address of FF-FF-FF-FF-FF-FF. To allow all the nodes in the server farm to receive the broadcasts at Layer 2, e3 will need to be configured to forward directed broadcasts with the following command: RTA(config)#interface e3 RTA(config-if)#ip directed-broadcast 58

59 Configuring IP helper addresses L3 Broadcast L2 Broadcast Helper address configuration that relays broadcasts to all servers on the segment. RTA(config)#interface e0 RTA(config-if)#ip helper-address RTA(config)#interface e3 RTA(config-if)#ip directed-broadcast 59

60 IP address issues solutions This module has shown that IPv4 addressing faces two major issues: The depletion of addresses, particularly the key medium-sized space The pervasive growth of Internet routing tables In 1994, the Internet Engineering Task Force (IETF) proposed IPv6 in RFC 1752 and a number of working groups were formed in response. IPv6 covers issues such as the following: Address depletion Quality of service Address autoconfiguration Authentication Security It will not be easy for organizations deeply invested in the IPv4 scheme to migrate to a totally new architecture. As long as IPv4, with its recent extensions and CIDR enabled hierarchy, remains viable, administrators will shy away from adopting IPv6. A new IP protocol requires new software, new hardware, and new methods of administration. It is likely that IPv4 and IPv6 will coexist, even within an autonomous system, for years to come. 60

61 IPv6 Three general types of addresses exist: Unicast An identifier for a single interface. A packet sent to a unicast address is delivered to the interface identified by that address. Anycast An identifier for a set of interfaces that typically belong to different nodes. A packet sent to an anycast address is delivered to the nearest, or first, interface in the anycast group. Multicast An identifier for a set of interfaces that typically belong to different nodes. A packet sent to a multicast address is delivered to all interfaces in the multicast group. 61

62 IPv6 To write 128-bit addresses so that they are readable to human eyes, the IPv6 architects abandoned dotted decimal notation in favor of a hexadecimal format. Therefore, IPv6 is written as 32 hex digits, with colons separating the values of the eight 16-bit pieces of the address. 62

63 IPv6 IP v6, or IPng (IP the Next Generation) uses a 128- bit address space, yielding 340,282,366,920,938,463,463,374,607,431,768,211,456 possible addresses. 63

64 Summary This module described how all of the following could enable more efficient use of IP addresses: Subnet masks VLSMs Private addressing Network address translation (NAT) 64