Subnetting the subnet

RD-CSY2001-2008/09

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-CSY2001-2008/09

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-CSY2001-2008/09

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-CSY2001-2008/09

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-CSY2001-2008/09

RD-CSY2001-2008/09

RD-CSY2001-2008/09

Address format <IP address/prefix P>. The prefix denotes the upper P bits of the IP address. Can be used to specify arbitrary blocks of addresses◦ Say an ISP has 192.5.48.0, 192.5.49.0,192.5.50.0,192.5.51.0

then◦ the IP address advertised will be 192.5.48.0/22

An ISP can obtain a block of addresses and partition this further to its customers

Say an ISP has 200.8.4/24 address (256 addresses)◦He has another customer who needs only 4 addresses from

200.8.4.24 then that block can be specified as 200.8.4.24/30

RD-CSY2001-2008/09

RD-CSY2001-2008/09

Net A: 14 HOSTS

Net B: 28 HOSTS

Net D: 7HOSTS

Net E: 28 HOSTS

Net C: 2 HOSTS

Router A Router B

Given IP address 204.15.5.0/24, use VLSM to create a subnetting scheme

Please try other questions on VLSM, available on module web page

RD-CSY2001-2008/09