Data Networking Year 2

Subnetting

Colm Bennett

From Class to Subnet

Basic Class A, B, C was very wasteful of IP addresses

Also difficult to manage internally

In theory a company that needs 5000 IP adresses should get a Class B

But this wastes ~60,000 ip addresses

Plus they probably don't manage those 5000 as one network

So they tended to ask for multple Class C's

Harder to control internally

Starts to clog up routing tables on the internet

Partial Solution - Subnetting

A subnet is a sub network within one of the normal classful networks

Done by specifying a subnet ID within the host part of the IP address

So it robs some of the bits from the host part to use as a subnet ID

Specified as a custom Subnet Mask

Allows large Class A or B networks to be managed more efficiently internally

Subnetting

Obviously required protocols to understand this convention

RIP 2 for example

Designing a Subnet Solution

Establish the number of physical networks required

Subnet the network to give at least this number of networks

Confirm that the number of hosts left per subnet is OK

Design cont.

Subnet Ids of all 0's and all 1's are not normally used

Similar reasoning to Host Ids

So possibly number of subnets is 2^n 2 where n = number of bits taken from the host field

So a 4 bit subnet mask will allow for (2*2*2*2)-2 = 16-2 = 14 subnets

Class B

Possible Subnets You Do!

Example

Class B 153.74.0.0

Company has 50 physical networks, max 200 hosts on any one

Looking at chart, taking 6 bits would give 62 possible subnets

1022 Hosts per subnet so OK

Robbing 6 bits means 3rd octet of Subnet Mask becomes

1111 1100

= 128 + 64 + 32 + 16 + 8 +4 = 252

Subnet Mask = 255.255.252.0

Practical

Irish company has office in each county on the island

Need 6 hosts in each office

Will a Class C network do?

Partial Solution?

Only a partial solution

Solved the internal management issue

Stopped medium size companies requesting many Class C's

But still left those companies being assigned a Class B and wasting 1000's of IPs

Needed some way of moving beyond the whole Classful approach - CIDR/VLSM