Anyways, I wrote this because someone came by my office today asking if I could quickly explain supernetting. Though I’m not sure why, considering I don’t work in the network department (but I do harass the poor folks there an awful lot).
The term supernetting officially states something along the lines of: Combing two or more contiguous network addresses to make the appearance of a single usable network address. The third octet of the first network address in the series MUST be divisible by the amount of networks in question. Folow me? No? Okay, I’ll explain.
Network Address # 1: [b]200.66.54.0[/b]
Network Address # 2: [b]200.66.55.0[/b]
The third octet in the first network address is ‘54’, which is divisible by the number addresses in series (in our case… 2). If the number had been ‘57’, this wouldn’t have worked (57 isn’t evenly divisible by 2).
The process of subnetting is “stealing” bits from the Network ID for use in the Host ID. As you know, the default submask for a Class C address is 255.255.255.0, or 11111111.11111111.11111111.00000000 in binary. There are exactly 24 bits in the Network portion and exactly 8 bits in the Host portion. This allows for 254 usable addresses (256 – 2 for the network/broadcast addresses). However, we want more… more… MORE! Infact, we’re going to “steal” 3 bits from the Network portion and put it on the Host portion.
Standard subnet mask : (255.255.255.0) 11111111.11111111.11111111.00000000
New subnet mask : (255.255.248.0) 11111111.11111111.11111000.00000000
What does this all mean? Wwweeellllllll… As previously said, there are 8 bits in the Host ID under the standard subnet mask. In the new subnet mask there are 11 bits (11 zeros) to play with. How many hosts you ask? 2^11 = 2048. Yes that’s right, 2048 hosts masquerading as one. Keep in mind though the first address and the last address in the series are UNUSABLE as they are network/broadcast addresses respectively. So you could (as my ISP does) assign an address ending in .0 or .255 (providing they’re not the FIRST or LAST address in the supernet). It’s considered bad practice to assign 0/255’s though.
Lastly, you can only supernet through routing protocols that support the “subnet” mask field. The only two that come to mind are OSPF (Open Shortest Path First) and RIPv2.
Anyways, that’s my brief intro. Hope you enjoyed it, let me know what you thought (think12[at]o s i x[d0t]net). This is mirrored on libexec.net too.
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Copyright Open Source Institute, 2006 
