Subnetting (Part 1)

(Updated: )
IT

Overview

  • CIDR (Classless Inter-Domain Routing)

IPv4 Review

  • CIDR throws away Class A, B, C, D, E in IPv4.
Class First Octet (Binary) First octet range (decimial) Prefix Length
A 0xxxxxxx 0 - 127 0.0.0.0 - 127.255.255.255 /8
B 10xxxxxx 128 - 191 128.0.0.0 - 191.255.255.255 /16
C 110xxxxx 192 - 223 192.0.0.0 - 223.255.255.255 /24
D 1110xxxx 224 - 239 224.0.0.0 - 239.255.255.255
E 1111xxxx 240 - 255 240.0.0.0 - 255.255.255.255
However, only class A, B, C can be assigned to a device. The other 2 are reserved.

IPv4 Addresses and Assignment

  • The IANA (Internet Assigned Numbers Authority) assigns IPv4 addresses/networks to companies based on their size.
    • For example, a very large company might receive a class A or class B network, while a small company might receive a class C network.
    • This system can lead to many wasted IP addresses.
      • Point-to-point networks typically use class C networks.
      • Or perhaps a company has 5000 end hosts, which is too large for a class C, so a class B must be used, resulting in 60,000~ addresses wasted.

CIDR (Classes Inter-Domain Routing)

  • IETF (Internet Engineering Task Force) introduced CIDR in 1993 to replace classful addressing system.
  • The removal of class allowed for larger networks to be split into smaller networks, called ‘subnetworks‘ or ‘subnets‘

Consider the following

  • CIDR let’s us use different prefix lengths.

CIDR Practice

203.0.113.0/25 = 2^7 = 128 - 2 = 126

203.0.113.0/26

203.0.113.0/27

203.0.113.0/28

203.0.113.0/29

203.0.113.0/30

/30 is a subnet of the class C network.

  • 203.0.113.0/30 includes 203.0.113.0 - 203.0.113.3, 4 possible addresses, but only 2 usable addresses.

The remaining addresses in 203.0.113.0/24 address block (203.0.113.4 - 203.0.113.255) are now available to be used in other subnets!

Is there a way to be even more efficient beyond /30?

What’s so special about /31?

  • There are 0 usable addresses. However, for a point-to-point connection, it’s possible to use a /32 netmask.
  • 203.0.113.0/31 contains the addresses:
    • 203.0.113.0
      • 11001011.00000000.01110001.00000000
    • 203.0.113.1
      • 11001011.00000000.01110001.00000001
  • Such a configuration is valid for a point-to-point connection.
  • The remaining addresses in the 203.113.0/24 address block (203.0.113.2 - 203.0.113.255) are available to be used in other networks. Great.

and /32?

  • Unusable to configure interfaces.
  • Some uses include:
    • Static route to specify an exact host.
    • Other…

CIDR Notation (Class C)

Question 1

  1. Determine the number of networks required.
    1. 45 + 2 = 47
    2. 47 * 4 = 188 (well within the range of a class C subnet which has 256 subnets)
  2. How to calculate the subnets we need to make?
    1. /30 gives us 2^2 - 2 = 4 usable addresses
    2. /29 gives us 2^2 - 2 = 6 usable addresses
    3. /28 gives us 2^4 - 2 = 14 usable addresses
    4. /27 gives us 2^5 - 2 = 30 usable addresses
    5. /26 gives us 2^6 - 2 = 62 usable addresses – this provides us more than we need.

Subnet 1: 192.168.1.0/26

  • (range is 192.168.1.0 to 192.168.1.63), where…
    • 192.168.1.0 = network addresses
    • 192.168.1.63 = broadcast address
    • 192.168.1.1 = first usable
    • 192.168.1.62 = last usable
      Subnet 2: 192.168.1.64/26
  • (range is 192.168.1.64 to 192.168.1.127), where…
    • 192.168.1.64 = network addresses
    • 192.168.1.127 = broadcast address
      Subnet 3: 192.168.1.128 - 192.168.1.191
      Subnet 4: 192.168.1.192 - 192.168.1.255