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.

More Useful Computer and Internet Basics Articles:

✔ How do I blur my house on Google Maps Street View?

I was poking around on Google Maps looking at satellite views of my neighborhood and when I switched to street view, was upset...

✔ Create a custom vanity URL for Kickstarter?

I was reading some updates on Twitter and saw someone had posted a URL that would let me see what projects they'd backed...

✔ Export or Save Subscription List from Google Reader?

Just heard that Google Reader is going away this summer. That stinks! How am I supposed to read my RSS feeds? More importantly,...

✔ Shrink or Reduce a Photo File Size on Mac?

I'm trying to upload some photos to a social media site and it's complaining that they're too big. They are, as they come...

✔ Can I organize my Yahoo Mail with folders?

I've been on Yahoo Mail for years and while most of my friends are now on Gmail or their own Web-based email programs,...

Let's stay in touch!

Sign up for my weekly AskDaveTaylor Newsletter and you'll receive even more tech and gadget help right to your inbox, along with exclusive news and industry updates. It's good stuff. I promise!

Categorized: Computer and Internet Basics (Article 4278, Written by Dave Taylor)
Tagged:
Previous: Moving Documents and Settings folder onto second hard disk?
Next: What's the best business blogging tool?

Reader Comments To Date: 11

Kevin Bailey said, on November 16, 2005 6:01 PM:

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.

Jame said, on April 20, 2006 9:07 AM:

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......

Karl said, on March 5, 2007 4:44 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.

rafiullah said, on July 30, 2008 4:15 AM:

Dear Sir,

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

rafiullah

felipe said, on May 31, 2009 12:23 AM:

Helpful post. THanks so much.

naren said, on July 22, 2009 2:31 AM:

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......

Brandon said, on August 12, 2009 10:38 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.

kshitija said, on January 15, 2010 5:11 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.

marcos said, on February 4, 2010 10:15 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

uma said, on June 16, 2011 3:57 PM:

Hiya,
I'm studying comptia n+ and can't get to grips with the formula used fofr subnetting, please can you email me explaining to me in the dumbest possible sense how to use this wformula and how it works.

Could you make it so a primary school child can understand it, because all the NORMAL ADULT ways I have tried to learn it & understand it don't seem to work.

Think of me as your new project, "PROJECT DUMB ASS"

Thanks

Uma

monish said, on August 5, 2011 5:04 AM:

pls provide ,with an example that how do we do subnetting.
i will be very thank ful to you

I do have a lot to say, and questions of my own for that matter, but first I'd like to say thank you, Dave, for all your helpful information by buying you a cup of coffee!

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

How does a subnet work?

Reader Comments To Date: 11

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

Check This Out Too...