Subnetting the subnet

RD-CSY3021-2008/09

Company with 200 hosts

Assigned class C network 195.10.1.0/24

Want 6 different LANs

Subnet class C network using mask 255.255.255.224 (/27)

Whole network has the same subnet mask or supports equal number of hosts on all the sub-networks◦ Is this a problem?

◦ Why?

◦ What should be done to solve it ?

RD-CSY3021-2008/09

VLSM Subnets a subnet

◦ VLSM divides a network into subnets of various sizes to

prevent wasting IP addresses

Uses up wasted addresses

Conserves address space on WAN links

Point-to-point link only needs 2 addresses

With VLSM point-to-point links can use a /30 mask

RD-CSY3021-2008/09

Variable Length Subnet Masking allows division of address space based on the size of networks

◦ Start with network requiring the most addresses

◦ Create a subnet mask

(use CIDR – Classless Inter Domain Routing)

◦ Subnet the subnet as needed to provide address space required for other subnets

Be logical – start at beginning or end or address space

Addresses must be contiguous to enable route summarization

RD-CSY3021-2008/09

Question

If an ISP has a 200.1.1.0 Network, he wants to give

assign to 5 customers who need 20, 12, 6, 25, 18 hosts.

◦ What should the subnet mask be?

◦ What is the address range in each subnet?

◦ What should the routing table entries be?

◦ Can it be done efficiently to conserve IP addresses?

RD-CSY3021-2008/09

RD-CSY3021-2008/09

RD-CSY3021-2008/09

You are required to connect two networks in Brisbane

to those in Perth, having the following requirements:

Brisbane needs to support 25 hosts each on the two

networks

Perth office needs to support 5 and 10 hosts in its two

offices

You may use only 128.186.1.0 /24

RD-CSY3021-2008/09

Office #hosts #host bits #usable

hosts

subnet

Brisbane 25 5 30 /27

Brisbane 25 5 30 /27

Perth 10 4 14 /28

Perth 5 3 6 /29

WAN 2 2 2 /30

RD-CSY3021-2008/09

Sort the subnet requirements by size, largest subnet first.

For each required subnet, allocate address space out of the smallest available subnet

128.186.1.0/27

128.186.1.32/27

128.186.1.64/27

128.186.1.96/27

128.186.1.128/27

128.186.1.160/27

128.186.1.192/27

128.186.1.224/27

RD-CSY3021-2008/09