CCE1030 Computer Networking

Transcription

1 CCE1030 Computer Networking Lecture 19 Subnetting CIDR / VLSM Usama Arusi January 2018 CCE1030 Usama Arusi 1

2 Lecture Content Introduction Classful IP Addressing Classful Addressing Structure Classless IP Addressing CIDR Classless Routing Protocols VLSM January 2019 CCE1030 Usama Arusi 2

3 Introduction Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different usable classes (A, B and C), known as classful IP addressing Class A, B, & C addresses were designed to provide IP addresses for different sized organizations January 2019 CCE1030 Usama Arusi 3

4 Classful IP Addressing Classes of IP addresses are identified by the decimal number of the 1st octet Class A address begin with a 0 bit Range of class A addresses = to Class B address begin with a 1 bit and a 0 bit Range of class B addresses = to Class C addresses begin with two 1 bits & a 0 bit Range of class C addresses = to January 2019 CCE1030 Usama Arusi 4

5 Classful Addressing Structure An IP address has 2 parts: The network portion - Found on the left side of an IP address The host portion - Found on the right side of an IP address January 2019 CCE1030 Usama Arusi 5

6 Classful Addressing Structure Purpose of a subnet mask It is used to determine the network portion of an IP address Classful Routing Updates Classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing updates The reason is that the Subnet mask is directly related to the network address January 2019 CCE1030 Usama Arusi 6

7 Classful Addressing Structure IP address space was depleting rapidly the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR) CIDR (Classless Inter-domain Routing) uses Variable Length Subnet Masking (VLSM) to help conserve address space VLSM is simply subnetting a subnet January 2019 CCE1030 Usama Arusi 7

8 Classless IP Addressing Classless Inter-domain Routing (CIDR) is a way to allow more flexible allocation of Internet Protocol (IP) addresses than was possible with the original system of IP address classes. Advantage of CIDR: More efficient use of IPv4 address space. Reduced the problem of wasted address space Route summarization, making a flexible way to specify network addresses in routers January 2019 CCE1030 Usama Arusi 8

9 Classless Inter-domain Routing (CIDR) IP network is represented by a routing prefix known as CIDR notation address is written with a suffix indicating the number of bits of the prefix, such as /24 this is an indication of the length of the mask. Variable Length Subnet Masking (VLSM) Allows a subnet to be further sub-netted according to individual needs Prefix Aggregation a.k.a. Route Summarization CIDR allows for routes to be summarized as a single route January 2019 CCE1030 Usama Arusi 9

10 Classless Inter-domain Routing (CIDR) Route summarization: A hierarchical architecture where each domain takes its IP addresses from a higher level. For example 1: ISP owns network /16, then ISP can offer /24, / /24 to its customers by sub-netting ISP only needs to advertise /16 Example 2: ISP with customer networks /24 to /24 Advertise /20 Therefore reducing the size of routing table entry January 2019 CCE1030 Usama Arusi 10

11 Classless Inter-domain Routing (CIDR) Route summarization done by CIDR Routes are summarized with masks that are less than that of the default classful mask Example: / 13 is the summarized route for the / 16 to / 16 classful networks January 2019 CCE1030 Usama Arusi 11

12 Classless Inter-domain Routing (CIDR) Steps to calculate a route summary 1. List networks in binary format 2. Count number of left most matching bits to determine summary route s mask 3. Copy the matching bits and add zero bits to determine the summarized network address January 2019 CCE1030 Usama Arusi 12

13 Classless Routing Protocol Characteristics of classless routing protocols: Routing updates include the subnet mask Supports VLSM Supports Route Summarization January 2019 CCE1030 Usama Arusi 13

14 Classless Routing Protocol Feature RIP-1 OSPF RIP-2 Classful Yes No No Classless No Yes Yes Supports VLSM No Yes Yes Sends Subnet Mask No Yes Yes January 2019 CCE1030 Usama Arusi 14

15 Variable Length Subnet Masks (VLSM) VLSM & classless routing Interior routing within an enterprise. This is the process of subnetting a subnet Allowing for use of different masks for each subnet More efficient use of IP addresses as compared to classful IP addressing January 2019 CCE1030 Usama Arusi 15

16 Variable Length Subnet Masks (VLSM) VLSM the process of sub-netting a subnet to fit your needs Example: Network /24 require the following subnets with supporting hosts. Subnet # Hosts Subnet # Hosts Network ID Address range Broadcast A 14 B 28 B E / / C 2 A / D 7 D / E 28 C / January 2019 CCE1030 Usama Arusi 16