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New Member

IP and NAT

hi first of all i m bit confuse abt the command ip claassless........what does that do actually i have read in the book but didnt get the point......secondly a quick question abt if we have class c address (public) so the max host(non public) we can used would be what if we have more then 254 host on lan and all wants to access that would be the reason we will use PAT??? not sure abt the concept......thanks in advance

New Member

Re: IP and NAT


IP classless in used when for e.g At times, a router might receive packets destined for a subnet of a network that has no network default route.

Suppose a router in network connected to subnets,, and Suppose the host sends a packet to By default, if the router receives a packet destined for a subnet it does not recognize, the router discards the packet.Therefore, when the host sends a packet to, instead of discarding the packet, the router forwards the packet to the best supernet route.

On your second query for PAT,you are right.

Several internal addresses can be NATed to only one or a few external addresses by using a feature called Port Address Translation (PAT) which is also referred to as "overload", a subset of NAT functionality.

PAT uses unique source port numbers on the Inside Global IP address to distinguish between translations. Because the port number is encoded in 16 bits, the total number could theoretically be as high as 65,536 per IP address. PAT will attempt to preserve the original source port, if this source port is already allocated PAT will attempt to find the first available port number starting from the beginning of the appropriate port group 0-5111, 512-1023 or 1024-65535. If there is still no port available from the appropriate group and more than one IP address is configured, PAT will move to the next IP address and try to allocate the original source port again. This continues until it runs out of available ports and IP addresses.

I hope this helps...Kindly rate this post