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How does a subnet work?

I have a "friend" that has an embarrassing problem... his network is too large and he's running out of IP addresses. He has a 192.168.1.x network, and I understand that he could put some machines on a 192.168.2.x network but he would need a router between the two... however, I also believe that through subnet masking he can make all the machines (on both networks) communicate as if on the same network, so that they can still use network printers, and one password authentication server, etc. How do you make subnet masks work?

Dave's Answer:

First, a bit about IP subnetting. With your IP address, you have a subnet mask. This subnet mask determines which IP addresses are located on your local network. You also have a default gateway, which is the network device (typically a router or firewall) that the machine knows to contact for any machine that is not on the local network.

The most common subnet mask on private networks is 255.255.255.0, which means the last octet of the IP address, like 192.168.1.X above, can be 1 through 254. This means once you get above 254 devices on your network, you run out of address space on a network using a 255.255.255.0 subnet mask.

At this point, your options are to either change your subnet mask, or put in a router. Which is most appropriate depends on plans for network growth. In a fast growing network, or one with a significant amount of network traffic, a layer 3 switch (a switch that acts as a very fast router) is the most appropriate choice. In a network that won't get many more than 275-300 hosts and doesn't have a whole lot of network traffic, using a larger subnet mask is an acceptable choice.

To increase the number of IP's on the subnet, change the subnet mask on *all* hosts to 255.255.252.0, while still using the same IP addresses. Make sure every single host gets changed, or you will have communication issues. With a 255.255.252.0 mask, your network will now range from 192.168.0.0 through 192.168.3.254.

There is substantially more to IP subnetting and network design than could possibly be described here. Inappropriate subnetting and design can lead to severe network performance problems, so I strongly suggest if you don't have expertise in this area that you contact a qualified network consultant experienced in enterprise network design.

Suggested resources:

Thanks to Chris Buechler for his splendid information, as always.

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Comments

The network would not range from 192.168.0.0 to 192.168.3.254. The address 192.168.0.0 has all the host bits as zero (the last ten bits) and as a result is Reserved. It would actually range from 192.168.0.1 to 192.168.3.254.

Posted by: Kevin Bailey at November 16, 2005 6:01 PM

Dear Dave,
I am much like a rookie in networking but would like to know more about IT. I have been reading books but have questions don't know where to turn to. I have a quick question concerning IP Subnetting. I know there are many topic on the internet to read but I really need your recommendation in a short time.

My company currently conducting at three locations. Each location has its own win2k3 AD. Each location has differ subnet from each. Example: Location A: 192.168.10.x Location B: 192.168.20.x Location C: 192.168.30.x Each location is not running out of any IPs yet. I would like to know is this a good way of designing a network? Is this efficient in technique and perfomance?

I am able to connect using PPTP VPN to all three locations but no WAN links. Each location has database and file server. Please advise......

Posted by: Jame at April 20, 2006 9:07 AM

Recently I installed a virtual OS with that "gave" itself an IP address of 192.168.3.128 with a subnet mask of 255.255.255.0. My host machine (say one as the virtual machine is located on) has an IP address of 192.168.0.101 with a subnet mask of 255.255.255.0. I can share files between machines and log onto an application (running on the virtual machine) via a client program (running on the host machine) that "points" to 192.168.3.128. How is this possible? Default gateway = 192.168.0.1 / Dlink DI - 624 router.

Posted by: Karl at March 5, 2007 4:44 AM

Dear Sir,

Your are very grad person
Kindly sent us the sub net ip configration Ip
regards

rafiullah

Posted by: rafiullah at July 30, 2008 4:15 AM

Helpful post. THanks so much.

Posted by: felipe at May 31, 2009 12:23 AM

I am able to connect using PPTP VPN to all three locations but no WAN links. Each location has database and file server. Please advise......

Posted by: naren at July 22, 2009 2:31 AM

I am in a Network+ class and this subnet voodoo really had me scared, the first page I saw was this, and you may not have room for it, but this guy you may have heard of "Microsoft" has a great explanation http://support.microsoft.com/kb/164015

To all the "Net Admins" out there who are looking for advice online, I HIGHLY recommend the Comptia Network+ course.
Best of luck to all of us for the up and coming IPv6... that one makes me want to become a commercial fisherman.

Posted by: Brandon at August 12, 2009 10:38 AM

Dear Sir,

i am a final year student of computer science. can u please tell me how to make a working model on subnet. wat all things i should show to make it innovative. please help as soon as possible.

thnx.

Posted by: kshitija at January 15, 2010 5:11 AM

I have about 80 devices on a 2 switches that are on subnet 255.0.0.0. The rest of the house is on subnet 255.255.255.0. the router is on 255.255.255.0. I need a way to communicate to the 255.0.0.0 while connected to the 255.255.25.0.
What do i need to do, to get this going?

Thanks,
Marcos

Posted by: marcos at February 4, 2010 10:15 AM

I have something to say, now that you mention it, but ...

I do have a lot to say, and questions of my own for that matter, but first I'd like to say thank you for all your efforts on this Web site by buying you a cup of coffee!

I do have a comment, now that you mention it!