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Pastimes : Dream Machine ( Build your own PC ) -- Ignore unavailable to you. Want to Upgrade?

To: peter michaelson who wrote (8089)	7/12/1999 4:07:00 PM
From: Spots	Read Replies (1) \| Respond to of 14778

>>What does 'subnet mask' mean, anyway? It's what a subnet wears to a fancy-dress ball. BUT if you mean an IP subnet mask, that's a little different. An IP subnet specifies a collection of IP addresses that are locally connected (on the same subnet), that is, the addresses that can be reached without going through a gateway. Subnet masks are 32 bits and consist of 1's followed by 0's from left to right, that is, it's illegal to have a 1 after the first zero. The zeros of the mask specify the variable part of addresses in the subnet; the ones in the mask specify the fixed part. For instance, my local lan has a subnet mask of 255.255.255.248, which is 111...111000, and my assigned IP addresses are xxx.xxx.xxx.240 - 247. (I'm not telling the xxx's, but they're behind a firewall anyhow <g>.) So anything I send from a node on this lan going to xxx.xxx.xxx.240 through xxx.xxx.xxx.247 go to an address on the subnet as determined by the mask. It matches because it is the same as my address in all the bits that are 1 in the mask and differs at most in bits that are 0 in the mask. So IP sends it out on the local lan. If it doesn't match my machine's address in some bit that's 1 in the mask, IP sends it to a gateway for routing through the IP network (the gateway better have an address on my subnet). Here's another way to think about it (sorry you asked yet?): Your IP address is X, your mask is M, and you generate a packet to go to IP address Y. IP does a logical AND of the mask with X and compares that to the logical AND of the mask with Y, that is: (X AND M) compared to (Y AND M) If these are equal, X matches Y in the positions that are 1 in the mask and are on the same subnet. If not, they are not on the same subnet and the packet must be routed.