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VLSM and CIDRVariable Length Subnet Masking and

Classless inter-domain routing

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VLSM and CIDR

This session will extend your understanding of sub-networking and explore address aggregation

This session summarises pack 2 on the OU T228 eDesktop site

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VLSM and CIDR

•Whilst IPv4 is 32 bit and offers over 4 billion addresses, severe restrictions now exist •Some systems may have a network, which is being extensively used•The loss of the broadcast subnet is considered wasteful•VLSM (Variable Length Sub-Net Masking) has two advantages: -

•It extends the hierarchical structure of the network addressing scheme, •Allows you to retrieve wasted addresses and apply them to suitable networked systems

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VLSM and CIDR

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VLSM and CIDR

The basic rules for VLSM are exactly the same as those for sub-networking, •You can only borrow (subdivide) from the host portion of the subnet in question•You can only have even numbers of variable sub-networks•Binary 1’s are always to the left, there are no exceptions for VLSM•Binary 0’s are always to the right, again there are no exceptions•Binary 1’s always represent the number of sub-networks•Binary 0’s always represent the number of hosts in each variable sub-network•Borrowing is done with a 1, each 1 means that you double the number of sub-networks. The subnet mask is 32 bits, remember the dotted notation is only to make the information human understandable.

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VLSM and CIDR

An easy VLSM example!Create a subnet mask for 100.0.0.0 which will ensure it has 1500 sub-networks

100 .0 .0 .011111111 .11111111 .11100000 .00000000255 .255 .224 .0

It is class AYou must have 2048 sub-networks

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VLSM and CIDR

An easy VLSM example!Create a subnet mask for 100.0.0.0 which will ensure it has 1500 sub-

networks

• Now we want the 100th sub-network• And yes we are going to use subnet 0 (TMA tip, declare it, use it)• So we have the network id of 99 for the 100th sub-network (count from

0-99, is 100 networks)• 99 in binary is 01100011

Place it on the right hand side100 .0 .0 .011111111 .11111111 .11100000 .00000000

.00001100 .01100000 .00000000100 .12 .96 .0

Is our elected subnet

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VLSM and CIDR

An easy VLSM example!Create a subnet mask for 100.0.0.0 which will ensure it has 1500 sub-

networks

Now we want 100 sub-networks within the 100th subnet, which means we can have 128

So we simply start from our new position100 .0 .0 .0100 .12 .96 .0255 .255 .224 .011111111 .11111111 .11100000 .0000000011111111 .11111111 .11111111 .11000000255 .255 .255 .192

With 62 (64-2) hosts per subnetwork

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VLSM and CIDR

You must be aware, you can VLSM to the Nth degree and VLSM a VLSMed

network until you run out of addresses

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VLSM and CIDR

VLSM problems

• With 99.0.0.0 • Create a subnet mask for 100 sub-networks• Find the 50th (subnet 0 may be assumed)• Create a subnet mask for a further 50 sub-networks• Now find the 25th

• Create a subnet mask for a further 10 sub-networks• With 200.0.0.0

• Create a subnet mask for 5 sub-networks• What is the last sub-network?• How many VLSM sub-networks can be fitted into this

space

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VLSM and CIDR

CIDR (Classless Inter Domain Routing)• Commonly referred to as supernetting• Is used to summarise a range of network addresses into

one• Reduces the size of a routing table• Reduces the updates on the inter router link• Creation of one aggregate address • Is not an exact science, which means we can have more

than one answer to the same problem

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VLSM and CIDR

CIDR (Classless Inter Domain Routing)

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VLSM and CIDR

CIDR (Classless Inter Domain Routing)The process of supernetting is considerably simpler than sub-networking, as the desired outcome is to create an address and subnet mask which summarises a range of known (and previously sub-networked) network addresses.

To calculate a supernet address, you have to: -

•Line up all the network addresses•Compare the binary•Find the common ‘bit!’

Calculate the new aggregate address and mask based on the common bit, and that which is to the left of it

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VLSM and CIDR

CIDR (Classless Inter Domain Routing)

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VLSM and CIDR

CIDR (Classless Inter Domain Routing)

CIDR Exercise

For …

80.16.10.080.16.20.080.16.30.080.16.40.080.16.50.0

Find a suitable summary address

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VLSM and CIDR

CIDR (Classless Inter Domain Routing)

CIDR Exercise

For …

80.16.0.080.26.0.080.56.0.080.66.0.080.76.0.0

Find a suitable summary address

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VLSM and CIDR