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What is the significance of a subnet value of 192.168.1.xxx compared to 192.168.2.xxx for a router?

Posted on 2007-12-01
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Hi Everyone:

       I often here and read of the word "subnet" within the realms of networking and routers.  For instance, there are ranges for routers which I have seen which can go from 192.168.1.xxx to 192.168.2.xxx.  Of course, I realize that 192.168 are fake IP addresses given by the router.  And, if I understand it correctly, the "1" represents a subnet and the 2 represents a subnet.  The last set of digits are unique IP values given by the router to each computer on the network as long as DHCP is enabled.  Also. the last set of digits, which are different for each pc, is used like an ID system so the router knows the location of sending data to a pc requesting information from it.  

           Basically, I am wondering why some routers are 192.168.1.xxx while some are 192.168.2.xxx, thus, using different subnets.  

           Any shared input on this question will be appreciated.

          Thank you

          George
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Question by:GMartin
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by:multithreading
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It's the subnet mask that defines the subnet. If your mask is 255.255.0.0 (or some other mask that excludes the last two bits of the third octet), then 192.168.1.xxx and 192.168.2.xxx are in the same subnet. If your subnet mask is 255.255.255.0 (which is more typical) then 192.168.1.xxx and 192.168.2.xxx are in different subnets.

Different subnets can be set up because of different firewall requirements, or to be served by different DHCP servers on different segments, etc.

If 1 and 2 are on the same subnet (subnet mask something like 255.255.0.0) , it could be just for keeping track of something like DHCP allocated addresses versus manually allocated addresses, or even part of a migration from one DHCP server to another. It is for some kind of infrastructure purpose, and shouldn't affect you.
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by:GMartin
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Hi

       Could you give me an example of a network setup in which two or more pcs occupy two different subnets, like 192.168.1.xxx and 192.168.2?  For instance, if a DSL modem has DHCP functionality which is enabled and a wireless router which of course has DHCP enabled, would two different subnets be needed in order to keep DHCP active in both?

         Thank you

         George

       
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Hey George,

Okay, let's try to break this down a bit.  
First, to clarify, 192.168.1.xxx and 192.168.2.xxx are not exactly "subnets".  Generally, when speaking about IP Version 4, which is what we are dealing with here, an IP Address is represented by a few parts.  We have a Full Network Address, take for example, 192.168.100.1.  Then we define a Subnet Mask.  IPV4 convention says that the default subnet for a class C IP Address (192.*.*.*) should be 255.255.255.0.

Okay, so let's take another look at those two parts, together.  Knowing that the subnet mask is 255.255.255.0 tells us the following: the network portion of an IP Address will come from the first 3 Octets of the Full Network Address (where the 255s are), and the Host portion comes from the last octet (where the 0 is).  So, the Network Portion of that Full Network address is 192.168.100.0, while the Host Portion is 0.0.0.1.

Putting those two pieces back together by doing some basic addition here 192.168.100.0 + 0.0.0.1 = 192.168.100.1.  This is an IP address.

Now, to answer your question:  If one group of IP addresses is 192.168.1.xxx and a second group of IP address is 192.168.2.xxx, with a subnet of 255.255.255.0, they will be on 2 different networks (as defined by the subnet address, which said that the network address is to come from the first 3 octets.  So computers with ip addresses of 192.168.2.52, 192.168.2.64, 192.168.2.15 will all communicate with each other on the same network.

Hope I answered your question, if not, let me know what else you'd like to know! Have a great night!
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by:GMartin
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Hi There;

        Thank you so much for the followup.  I found your explanation very insightful and well thought out.  With everything discussed so far, I am wondering what the maximum number of computers which can occupy a single subnet.  Is it 255?

        George
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Well, there are many different subnets that can be created.  Let me explain a little more:

A subnet of 255.255.255.0 says that there is 1 network on the network 192.168.1.xxx (So all 254 hosts, from 192.168.100.1 to 192.168.100.254 would be on 1 network. [You cannot use .255]) This example gives you 254 maximum computers.
If you used for example, 255.255.255.128 as the subnet mask, you now create 2 networks, which can each have 126 computers on them, yielding 252 hosts.  You lose 2 because the first and last host of each subnet (192.168.100.0, 192.168.100.127, 192.168.100.128, and 192.168.100.255) cannot be used, as these become the network starting and ending addresses.

Those were all examples of class C subnet masks.

So, to answer your question, the most computers that can occupy a single class C Subnet mask is 254.
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by:GMartin
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Hi There;

         Thanks so much for the followup.  Everything  you have presented certainly makes sense and certainly helping me to clear up some misconceptions as well as gaining newer insights into the significance of the actual addresses.  I noticed you give reference several times to "hosts".  When this term is used within the explanations, are we referring to computers which are receiving information from a DHCP server like a router?

          George
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Sorry, it's a bit of networking lingo.  Any device that communicating on the network via the TCP/IP Protocol and that is connected to the network must have an IP address.  Therefor, I used the word "host" just to clarify that it is not only computers that can take up IP addresses.  Any TCP/IP level device, such as a router, print server, computer, video game console, access point, server, etc is known in networking terms as a host.  

An example:  
I said above that you can have 254 hosts.
Sample Network: 192.168.100.xxx
Subnet Mask: 255.255.255.0
Network Address: 192.168.100.0
Network Ending Address: 192.168.100.255
Available IP Addresses: 192.168.100.1-192.168.100.254
254 IPs available for "hosts".
192.168.100.1 Could be a router
192.168.100.2 Could be a computer
192.168.100.3 Could be a print server
192.168.100.4 could be another computer

In your question you said "are we referring to computers which are receiving information from a DHCP server like a router?"

Answer: We are referring to computers (AND ANY OTHER DEVICES) which are using IPs on a network, INCLUDING a router.
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Hi George,

Looking over this discussion it's not clear you understand exactly what's being done by the subnet mask.   Remember that each of the 4 numbers in an IP address represents an 8-digit binary number (that's why they range from 0 to 255).   If you write the number in binary (e.g. 255 = 11111111) then each of the 1's represents a "mask" bit that must be matched for an address to be considered part of the same subnet.   Mathematically, an address is AND'd against the mask; and then the results of this are exclusive-Or'd (XOR'd) against each other to determine whether or not two addresses are in the same subnet.   If the result of the XOR operation is all zeroes, the addresses are in the same network.    Sounds complex ... but logically it's very simple ==> the 1's in a mask represent the binary digits that matter (must match);  the 0's represent the digits that don't matter (i.e. any value is okay).

Note that while 254 is the right answer for the # of devices in a subnet defined by a mask of 255.255.255.0 (a network that uses this mask is a "Class C" network), it is not correct for all subnets.   For example, with a subnet mask of 255 255 0 0 you could have 65,534 devices.   RFC 1918, which governs IP allocations for private networks, allows the 255.255.0.0 mask for the 192.168.x.x range of addresses;  but it also allows a 255.0.0.0 mask for the private 10.x.x.x network => so with this you could have up to 16,777,214 devices [probably more than you'll ever need :-) ]    Note that the "class" of a network is defined by the subnet mask => a Class A network has a subnet mask of 255.0.0.0; a Class B network uses 255.255.0.0; and a Class C network uses 255.255.255.0

As for your fundamental question here ("... Basically, I am wondering why some routers are 192.168.1.xxx while some are 192.168.2.xxx, thus, using different subnets.") ==>  There's no technical reason to use different ranges;  manufacturers do this to differentiate themselves [i.e. Linksys usually uses 192.168.1.x whereas NetGear typically uses 192.168.0.x -- although both use different ranges for their VOIP products (Linksys uses 192.168.15.x)].   But it doesn't matter => you can change the default IP for any router in its setup page;  and can specify the range of addresses it will assign via DHCP -- or even turn off the DHCP function altogether (which you would need to do, for example, if you have multiple routers in your network).

Basically, you have the right idea ==> the IP address is just an address; and the "subnet" defines the "neighborhood" that a particular range of addresses is within.

By the way, your comment "... I realize that 192.168 are fake IP addresses given by the router ..." is not correct.   There's nothing "fake" about the IP addresses ==> they are "private" IP addresses ... not "fake" :-)    RFC 1918 defines three ranges of private addresses:  10.0.0.0 - 10.255.255.255;  172.16.0.0 - 172.31.255.255; and 192.168.0.0 - 192.168.255.255 ==> most router manufacturers use the latter range.   But there's nothing "fake" about the addresses :-)
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by:GMartin
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Hi Everyone;

         Thanks so much for the shared input to this question.  Personally, I found each response instrumental in helping me to better understand the significance of the IP numbers for pc's and routers in addition to making sense out of subnet mask numbers.  

          Many thanks everyone for the great help on this one.  As always, I learned much from this thread.

           Thank you.

           George
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