Andrew Smith 1 VLSM and CIDR Variable Length Subnet Masking and Classless inter-domain routing

Andrew Smith 2 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

Andrew Smith 3 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

Andrew Smith 4 VLSM and CIDR

Andrew Smith 5 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.

Andrew Smith 6 VLSM and CIDR An easy VLSM example! Create a subnet mask for which will ensure it has 1500 sub-networks It is class A You must have 2048 sub-networks

Andrew Smith 7 VLSM and CIDR An easy VLSM example! Create a subnet mask for which will ensure it has 1500 sub- networks Now we want the 100 th 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 100 th sub-network (count from 0-99, is 100 networks) 99 in binary is Place it on the right hand side Is our elected subnet

Andrew Smith 8 VLSM and CIDR An easy VLSM example! Create a subnet mask for which will ensure it has 1500 sub- networks Now we want 100 sub-networks within the 100 th subnet, which means we can have 128 So we simply start from our new position With 62 (64-2) hosts per subnetwork

Andrew Smith 9 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

Andrew Smith 10 VLSM and CIDR VLSM problems With Create a subnet mask for 100 sub-networks Find the 50 th (subnet 0 may be assumed) Create a subnet mask for a further 50 sub-networks Now find the 25 th Create a subnet mask for a further 10 sub-networks With 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

Andrew Smith 11 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

Andrew Smith 12 VLSM and CIDR CIDR (Classless Inter Domain Routing)

Andrew Smith 13 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

Andrew Smith 14 VLSM and CIDR CIDR (Classless Inter Domain Routing)

Andrew Smith 15 VLSM and CIDR CIDR (Classless Inter Domain Routing) CIDR Exercise For … Find a suitable summary address

Andrew Smith 16 VLSM and CIDR CIDR (Classless Inter Domain Routing) CIDR Exercise For … Find a suitable summary address

Andrew Smith 17 VLSM and CIDR