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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
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