Understanding IP subnets and routing


IP networking and routing is not new to me, but each time I come back to it after a gap of a year or two, I find myself blundering through the same concepts. This post is my attempt to remind myself each time that happens.

IP version 4 uses a 32 bit address, represented as four octets in the decimal notation, separated by dots, e.g. 192.168.1.1. This implies that we can provide a unique address to more than four billion computing devices. Assuming for the moment that 4 billion addresses is enough, we cannot connect all those four billion computers in a single network. The bandwidth of the medium would have to be infinite, iterconnections would have to be very long, each computer would have to receive and discard a lot of information not destined for it and lots of other physical limitations.

The quite obvious solution is to break the problem down, which means creating several small networks and somehow iterconnecting them. To do that we bring into existence an entity called the router, which has a physical presence in one or more sub-networks and can selectively take data from one network to another. We also divide an IP address into two parts, the network address and the host address. This division is stated by using a subnet mask.

The immediate advantage of this scheme is that traffic destined to a subnet is seen only by hosts within that subnet, this is good for security and for scaling the network.

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