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

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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

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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

subnets Hosts available networks

1.0/27 32 1/8 Brisbane

1.32/27 32 1/8 Brisbane

1.64/28 16 1/16 Perth

1.80/29 8 1/32 Perth

1.88/30 4 1/64 WAN

1.92/30 4 1/64

1.96/27 32 1/8

1.128/25 128 1/2

RD-CSY3021-2008/09

RD-CSY3021-2008/09

Brisbane Perth

Please attempt the task on VLSM posted on module web page

RD-CSY3021-2008/09