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IP and subnet mask question?

Hi !

can someone explain me, how to calculate a subnet mask for a range of IP ?

and what is the next one : ex. IP address 12.11.10.9, with a subnet mask of 255.254.0.0 (the same as 12.11.10.9/15). ok, i don't understand what that /15 is ? 12.11.10.9 is the IP, but /15 ?

can someone tell me please ? thanks !

Relevance
• 1 decade ago

You can't calculate a subnet mask from just a range of ip addresses. What exactly are you trying to work out?

I can answer the /15 bit for you though. The first thing you need to know is that the subnet mask is a denary (base 10) representation of a binary number (base 2). ie 255 in base 10 = 11111111 in binary.

A single host has a mask of 255.255.255.255, which in binary is 11111111.11111111.11111111.11111111. This is written /24 as there are 24 1's and it's easier to read. So /15 would be 11111111.11111110.00000000.00000000.

The subnet mask tells you which part of the IP address is the host and which part is the network. If we convert the ip you've specified into binary and then write the subnet mask underneath 12.11.10.9 would become:-

00001100.00001011.00001010.00001001

11111111.11111110.00000000.00000000

So the first 15 digits of your ip address specify your network and the last 17 digits specify the individual host.

You can do lots more with this but it depends what you're trying to work out. This is a big topic, bigger than it seems.

• 1 decade ago

Subnet mask and IP address is really a binary number. for example the subnet mask 255.254.0.0 in binary is really:

11111111.11111110.00000000.00000000 while the IP address of

12.11.10.9 is: 00001100.00001011.00001010.00001001

The /15 part simply means that the 1st 15 bits of the IP address are the network address and all bits after that are the Hosts address. so saying 12.11.10.9 / 15 is the same as giving a IP address with a subbnet mask of 255.254.0.0. Above I also converted the subnet mask to binary. You will see that the 1st 15 bits are all "1's" which would also signify that the 1st 15 bits of the IP address are the Network address and the rest are the Host. So giving a subnet mask or using the /x notation is just 2 different ways of conveying the same info.

• 1 decade ago

Here is a link for a subnet calculator

http://www.t1shopper.com/tools/calculate/ip-subnet...

• 1 decade ago

it can be the server port number