TCP addresses and Net Masks

[Will Johnson]

First off, does anyone have any recommendations for a good TCP/IP book
written in plain english?

Second, I have a Novell 4.1 network. I also have 6 different subnets
on this network. Here's their addresses:

172.16.1.2 -- 172.16.1.95
172.17.1.2 -- 172.17.1.95
172.18.1.2 -- 172.18.1.95
172.19.1.2 -- 172.19.1.95
172.20.1.2 -- 172.20.1.95
172.21.1.2 -- 172.21.1.95

Not knowing any different, I used a net mask of 255.255.255.0.

Now that I am trying to add a gateway to one subnet. It works fine
for that subnet, but not the others. A brief explaination of how
subnets are calculated would be greatly appreaciated.

Thanks.
Will Johnson

[Barry Margolin]

In article <34bf84be...@news.midusa.net>,

Will Johnson <wi...@lhd.com> wrote:
>Second, I have a Novell 4.1 network. I also have 6 different subnets
>on this network. Here's their addresses:
>
>172.16.1.2 -- 172.16.1.95
>172.17.1.2 -- 172.17.1.95
>172.18.1.2 -- 172.18.1.95
>172.19.1.2 -- 172.19.1.95
>172.20.1.2 -- 172.20.1.95
>172.21.1.2 -- 172.21.1.95
>
>Not knowing any different, I used a net mask of 255.255.255.0.
>
>Now that I am trying to add a gateway to one subnet. It works fine
>for that subnet, but not the others. A brief explaination of how
>subnets are calculated would be greatly appreaciated.

The network mask tells you which parts of an address indicate the network
that tha host is on. A mask of 255.255.255.0 means that the left three
bytes of an address are the network, while the rightmost byte is the host
address within that network.

Hosts use this to determine whether to send something directly on the
ethernet or forward it to a router. They compare the destination address
with their own address, by looking at the network parts. So if a host with
address 172.16.1.10 is trying to get to a destination 172.16.1.20, it sees
that they both have the same network 172.16.1, so it sends there directly;
if it's trying to get to 172.20.1.30, it sees that 172.16.1 is not the same
as 172.20.1, so it sends it to a router.

--
Barry Margolin, bar...@bbnplanet.com
GTE Internetworking, Powered by BBN, Cambridge, MA
Support the anti-spam movement; see <http://www.cauce.org/>
Please don't send technical questions directly to me, post them to newsgroups.

[John D Loop]

Will,
The best I have found is "Networking with Microsoft TCP/IP", by Drew
Heywood. Lots of good stuff (certainly not just usoft).
John
Will Johnson wrote in message <34bf84be...@news.midusa.net>...

>First off, does anyone have any recommendations for a good TCP/IP book
>written in plain english?
>

>Second, I have a Novell 4.1 network. I also have 6 different subnets
>on this network. Here's their addresses:
>
>172.16.1.2 -- 172.16.1.95
>172.17.1.2 -- 172.17.1.95
>172.18.1.2 -- 172.18.1.95
>172.19.1.2 -- 172.19.1.95
>172.20.1.2 -- 172.20.1.95
>172.21.1.2 -- 172.21.1.95
>
>Not knowing any different, I used a net mask of 255.255.255.0.
>
>Now that I am trying to add a gateway to one subnet. It works fine
>for that subnet, but not the others. A brief explaination of how
>subnets are calculated would be greatly appreaciated.
>

>Thanks.
>Will Johnson
>
>
>

[Peter Van Oene]

John D Loop wrote:

> Will,
> The best I have found is "Networking with Microsoft TCP/IP", by Drew
> Heywood. Lots of good stuff (certainly not just usoft).

>

For TCP/IP books I believe 'Internetowrking with TCP/IP' Vol I from Comer
isgenerally regarded as one of the best.

.--
Peter Van Oene
Systems Engineer
Unis Lumin Inc.
van...@rogers.wave.ca

[Bob Vance]

> how subnets are calculated
I'm not sure what you mean here.
But, each bit in the subnet mask is 'and'ed with the corresponding
bit in the address. The result is the network id for routing
purposes.
You legally have network
172.16.0.0 /12
to play with (plus the others defined in RFC1918). Note,
/12 = 12 bits of prefix as the subnet mask, which gives you in the
old, "Class B" terms of 16-bit prefix, all the 16-bit networks
172.16-31.0.0
from which you dutifully chose.

And, you chose to subnet further to 24 bits:
172.16.1.0 /24
172.17.1.0 /24
172.18.1.0 /24
172.19.1.0 /24
172.20.1.0 /24
172.21.1.0 /24
Unfortunately, with a subnet mask of 255.255.255.0 (prefix /24 ) you are
limited to 254 devices on each subnet.
If you changed to prefix /16 bits (mask 255.255.0.0) (which is legal as
long as your second byte is 16-31), your networks would be:
172.16.0.0 /16
172.17.0.0 /16
172.18.0.0 /16
172.19.0.0 /16
172.20.0.0 /16
172.21.0.0 /16

each could support 256*256-2 devices.
Note that
172.16.1.0
would now denote a node on network 172.16.0.0/16 with
a node id of
1.0

Hope this helps.

-----------------------------------------------
Tks | bo...@sbm.com
BV | bobv...@alumni.caltech.edu
Bob Vance
VP Technical Consulting, SBM
Vox 770-623-3430 11455 Lakefield Dr.
Fax 770-623-3429 Duluth, GA 30097-1511
===============================================