Setting up subnet mask

Hi folks,

I'm running several computer labs, but all computers and printers are on the same subnet.  The problem is, computers in Lab A (for example) can "see" computers in Lab B (especially true in Mac).

And I'm wondering if there is any way to separate them by further dividing the subnet via changing subnet mas (right now, it's  Each lab has a series of consecutive IP addresses.  For example,

Lab A uses from to, and
Lab B uses from to, etc.

Any help will be appreciated!  ~elpmet
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gandalf94305Connect With a Mentor Commented:
You can always run more than one subnet on a single physical network. Just use disjoint ranges of addresses.


Lab A - to 30, subnet mask broadcast

Lab B - to 62, subnet mask broadcast

Lab C - to 94, subnet mask broadcast

and so on...

Using a network sniffer, each machine will still be able to see the other machines' traffic, however, in normal operation this will not be the case.

For machines in Lab A to talk to the other Lab B and Lab C machines, you will need a router. The easiest way would be to assign three IP addresses, one from each network, to a special machine which would also be defined as the (possibly default) router for each of the lab networks.

Ah, just in case you were wondering why I used the magic numbers 1..30, 33..62, etc.:

The subnet mask defines which bits belong to the network and which to the host part of an IP address. Thus, (FFFFFFE0) will leave the last six bits for the host part (123 is hexadecimal 7B):

xxxx7B01..xxxx7B1E (...1 to ...30) gives you a network mask of FFFFFFE0. The host part all one bits is the broadcast address, i.e., for that network: xxxx7B1F (...31).

You cannot have boundaries after 36 hosts. The number of hosts in a subnet is limited to a power of two minus two. Possible numbers are 2, 6, 14, 30, 62, 126, 254, ...

Use the subnet mask  This will give you a 4 possible subnets with 62 useable IP addresses per subnet.  For example, if your existing subnet is: through now;

You would then have: through through through through

The first and last IP addresses in each subnet are not useable because they are reserved for the network number and broadcast address for the subnet, respectively.

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All of these suggestions assume that you run TCP/IP only. I doubt that is the case and I'll bet NETBEUI and APPLETALK, possibly IPX/SPX are also running (enabled by default install on Windows 9x/NT). Changing the IP subnet mask will not make a difference. You will have to disable NETBEUI and IPX/SPX everywhere before even considering changing the subnets. Then you have to decide if there is ever a need to communicate between labs. In that case you will need something to route the packets between subnets.
Have any of these comments been of any help to you? Do you need more information?

The example in you proposed answer is incorrect. and are technically NOT valid subnets.  The first one ( can be used with IP Subnet Zero enabled on Cisco routers.  However the subnet is invalid as the address can not be both the network broadcast address for (Since this is a Class C address) and the subnetwork broadcast address for

Hope this helps.
Look at the subnet mask.  It is /26 or  This gives you the following subnets.

And the subnet IS a valid subnet address.  And the broadcast address IS  For the subnet, the broadcast address is  I am suggesting using the /26 subnet mask for the whole network, not for one subnet.

Get a subnet calculator.

I do not need a subnet calculator and I did not say's subnet broadcast address is the  I did say that is a Class C address and as such, is the network address and is the network broadcast address for the whole class C.  Having that been said, a network broadcast address and a subnetwork broadcast address can NOT be the same IP.  Therefore, is not technically a valid subnet since the subnetwork broadcast address would be

Get a book on IP.
As of IOS version 12.0, Cisco routers now have ip subnet-zero enabled by default. You can use the no ip subnet-zero command to restrict the use of subnet zero addresses.
Can I throw my two cents in?
You're both right.
Strict RFC classful behavior dictates that subnet zero and the broadcast subnet are not useable.
CIDR standards change that.
Cisco fully supports CIDR with "subnet zero" and "ip classless" commands in the IOS.
Microsoft does not adhere to the RFC's
Apple does
Novell ??
I don't think we can make the assumption that this network in question with its mix of operating systems and network OS's will fully support anything other than strict classful standards.


I completely agree with you.  And the reason behind the use of the "not technically valid" statement.
No comment has been added lately, so it's time to clean up this TA.
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lrmoore, look for your question in this topic area

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