How do you know which subnet mask to use when constructing a network? For instance do you give a router any available subnet such as 255.255.255.0 and the PC the next available subnet such as 255.255.0.0? Do you give it any address or is there a way to calculate this?

There is a way to calculate the ip ranges and subnets.
Take a look at this:
http://www.subnetmask.info/

Paul

for a small home/office network, you will only be using class boundary subnet masks ie 255.255.255.0 / 255.255.0.0 or 255.0.0.0.

the correct one to use will depend on the IP address range that you are using. This will undoubtedly be a private range ie 192.168.0.0 / 172.16.0.0 / or 10.0.0.0.

The correct subnet mask is matched in the order they are typed above. ie 192.x.x.x is 255.255.255.0 etc.

The host numbers that are added to the addresses determine the names of individual devices on your network ie 192.168.1.1 might be your cable modem/router/switch device, then 192.168.1.2 might be your PC and so on. its the network part (ie 192.168) that determines the mask.

-D-

nosey aren't ya, readin me sig?

More examples that are usually used on home network:

Example 1:

I have 5 computers and a router in my network. I will assign 10.0.0.1 to the router, 10.0.0.2 – 10.0.0.6 to other 5 computers. I use netmask 255.255.255.0 for this network, so that I can assign IP addresses 10.0.0.1 - 10.0.0.254 in the network. Network address is 10.0.0.0, broadcast address is 10.0.0.255.

Example 2:

I have 8 computers, 2 notebooks and a router in my network. I will assign 172.16.10.1 to the router, 172.16.10.2 – 172.16.10.9 to other 8 computers and 172.16.10.10 – 172.16.10.11 to other 2 notebooks. I use netmask 255.255.255.0 for this network, so that I can assign IP addresses 172.16.10.1 – 172.16.10.254 in the network. Network address is 172.16.10.0, broadcast address is 172.16.10.255.

Example 3:

I have 8 computers, a router and a network printer in my network. I will assign 192.168.1.1 to the router, 192.168.1.2 to the network printer and 192.168.1.3 – 192.168.1.10 to other 8 computers. I use netmask 255.255.255.0 for this network, so that I can assign IP addresses 192.168. 1.1 – 192.168.1.254 in the network. Network address is 192.168.1.0, broadcast address is 192.168.1.255.

Hope helps..

picohat,

Given those 3 examples wouldn't it have been better and more appropriate to use the proper mask for class full addressing rather than the subnetted class A & B networks?

Class Full Private IP address Ranges

Class A IP's: 10.0.0.0 – 10.255.255.255
Class A mask: 255.0.0.0

Class B IP's: 172.16.0.0 – 172.31.255.255
Class B mask: 255.255.0.0

Class C IP's: 92.168.0.0 – 192.168.255.255
Class C mask: 255.255.255.0

Using a subnetted class A or B for a home network doesn't make much sense to me and I've never come across a SOHO router that supports it, but then again I haven't used every available brand/model. Have you?

Its pretty sinmple when you think of subnet masks. Just ask yourself this question:

"How many computers to I need to network"?

254 or fewer computers, use 255.255.255.0
65,536 or fewer computers, use 255.255.0.0

I see no reason to ever need to use 255.0.0.0, but if you have 16,777,216 computers, you shouldn't be asking this question. hehehe

John Carr
Network Analyst

I would never use more than a /24 mask or /23 at most . It makes way too big a broadcast domain if you go bigger than this then you get to spend all your time listening to user saying the network is slow . it is good practice to keep your broadcast domain small , a /24 is a very good compromise . If you just need a few users on a subnet break it down further at the /25 or /26 . Good place to learn all this is at www.learntosubnet.com . If you are going to be in networking it is a must to understand all this .