TCP IP

www.learntosubnet.com

Go to this site and read up on it. Subnet mask is crucial for IP to communicate.

Go to the URL previously mentioned to know all about subnet masks. But the simple explaination is that the subnet mask tells the system how to divide up the network IP addresses.

255 is used to mean that the IP byte must match, 0 basically meansthat it is any address in that byte, and a specific number menas anything under that IP address in that BYTE.

Example

4 PCs with IP addresses of

10.1.5.25
10.1.5.26
10.1.7.36
10.2.3.45

With a mask of 255.255.255.0 the first two would be seen as part of the same network. The others would be seen as another network.

With a mask of 255.255.0.0 the first three would be seen as being part of the same network. And only the fourth would be seen as a different network.

And a mask of 255.0.0.0 would see tham all as part of the same network.

Assuming all these addresses belong in the one company you could limit interaction with the mask 255.255.255.0 thus the first two, sya in sales, could readily talk to each other. Whilst the third can be in accounting and the fourth in stores. thus each subnetwork would require validation etc to access across to the other subnetworks. This segregation helps to reduce the amount of traffic within a subnetwork.

Hope this helps you tounderstand the basics.

IP addresses and subnet masks show in 'dotted decimal notation' for ease of use. Computers read them in binary, and the mask sets out how many bits of the IP address are used in the Network Address and how many in the Unit Address, this is donw with a string of 0s.

The 24 bit subnet mask of 255.255.255.0 in binary as 11111111.11111111.11111111.00000000 the computer looks for a match with the 1s and takes anything for the trailling 0s.

With a lower level subnet,say 28 bits the binary mask is 11111111.11111111.11111111.11110000 - the computer reads the IP addresses for the last four bits as being anything belonging to that subnet, thus (concentrating on the last byte only)

11110001, 11110100, and 11111100 would be part of that subnet, whilst 11101111 would be seen as part of a different sub net.

It all revolves around the binary number and where the string of continuous 0s starts in the binary. Now the subnet mask of 255.255.255.0 is 11111111.11111111.11111111.00000000 whilst the subnet mask of 255.255.255.192 is 11111111.11111111.11111111.11000000 thus this would constitute part of the 255.255.255.0 mask. That is why when you set the subnet masks you must ensure that ALL computers in the system have the same mask entry or they get a conflict trying to work out who belongs in the network and who does not.

Most masks are often written in a shorthand that gives the number of bits after the mask eg 255.255.255.0/24 however this is not used where the mask is a full byte as it is assumed but a subnet in the last byte would be listed as 255.255.255.192/28

hope this helps, had originally assumed you were trying to set up a subnet as that is the most common reason for asking about masks.

BTW it is best to set up internal subnets using the reserved class A or B internal address ranges and use whole bytes for the divisions, it is easier to manage and allows for more expansion with less change later.

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