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Home > Archive > CCNA > October 2001 > Subnets?
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| Hello,
I had a discussion with a friend of mine about,do we consider each serial connection a SEPARATE subnet (sybex page 135 says yes as it considered a WAN connection)so if we do that how we deal with the serial interfaces (s0,1,2, and 3)and to which subnets they will belong?? are they considered a the gateway for the subnet they attached to? i am a bit confused, thank you for your input.(sorry for the poor diagram)
_________
subnet 1
.
.
E0
router A.
s0
.
.
T1
.
.
.
s1
router B.e1...........subnet 2
s2
.
.
T1
. .
s3
router C.
e2
.
.
subnet 3
_____
s = serial
e = ethernet | |
| dmaftei 2001-10-05, 9:45 pm |
| quote:
Originally posted by web
do we consider each serial connection a SEPARATE subnet
An interface belongs to a subnet if you configure an IP address on that subnet. A serial can be on one subnet, like this:
interface serial0
ip address 192.168.100.1
or on many subnets, like this:
interface serial0
ip address 192.168.100.1
ip address 192.168.101.1 secondary
ip address 192.168.102.1 secondary
...
or like this:
interface serial0.1
ip address 192.168.100.1
interface serial0.2
ip address 192.168.101.1
interface serial0.2
ip address 192.168.103.1
...
It's also possible to have unnumbered point-to-point links on serial interfaces, in which case the interface doesn't belong to any subnet.
Does this answer your question?
Cheers! | |
| depamo 2001-10-05, 11:58 pm |
| There are some other things that you need to consider to make this work but in general terms, all interfaces that talk to each other using Layer 2 addresses must be in the same subnet. You will find later that there are other technologies that allow a 'cloud' that requires multiple interfaces to exist within a subnet (one of these is Frame Relay).
For most cases that you will see right now, the network that exists between two serial interfaces in point-to-point connections must have their own subnet separate from other interfaces.
Usually you will use a 252 subnet mask to make this work which has 2 host addresses in it.
Overall, you the interfaces align with the TCP/IP requirements for communications. Traffic destined for a next hop router must be reachable by an interface on the router which must reside in at least one common subnet with the next hop router.
There are some other things you can do, such as have two routers on the same ethernet segment but this also follows the same rule of the two routers. If the interface is not in the same subnet correctly as the far side of the connection, the router will let you know every time it recieves a CDP packet.
So the other way, my interfaces exist with ip addresses common to the subnet of the ethernet segment on a different port, the router will also warn you against this. Having the same or subnet of a network that exists on a different segment without special briging considerations will cause routing problems and errors to show up.
Hope that explains it a little. Lots of concepts to get the full picture. | |
|
| depamo, this EXACT;Y WHAT I WAS TRYING TO SAY. The fllowing is copied from your post,
"For most cases that you will see right now, the network that exists between two serial interfaces in point-to-point connections must have their own subnet separate from other interfaces."
My question now is HOW? | |
|
| WOO HOOO that wasnt I passed post! thanks for those who tried. | |
| dmaftei 2001-10-06, 6:38 pm |
| quote:
Originally posted by web
"For most cases that you will see right now, the network that exists between two serial interfaces in point-to-point connections must have their own subnet separate from other interfaces."
My question now is HOW?
(eth0)RouterA(ser0)-------(ser0)RouterB(eth0)
RouterA:
...
interafce ethernet0
ip address 172.16.0.1 255.255.0.0
...
interface serial0
ip address 192.168.0.5 255.255.255.252
...
RouterB:
...
interafce ethernet0
ip address 10.0.0.1 255.0.0.0
...
interface serial0
ip address 192.168.0.6 255.255.255.252
...
RouterA's ethernet0 is in subnet 172.16.0.0/16, RouterB's ethernet0 is in subnet 10.0.0.0/8, both serials are in subnet 192.168.0.4/30. Makes sense? | |
|
| Excellent,YES it does, and thx alot. Counting on your patience (and knowledge)two more questions, are we doing that (putting the 2 serials on one separate subnet) because they represent a "point to point" connection? second question is taking router A as an example does 192.168.0.5 / 255.255.255.252 considered the gateway address for all hosts attached to e0 (and ofcourse the same apply to router B)?
Thanks alot. | |
| doctorcisco 2001-10-07, 10:23 am |
| quote:
Originally posted by web
Excellent,YES it does, and thx alot. Counting on your patience (and knowledge)two more questions, are we doing that (putting the 2 serials on one separate subnet) because they represent a "point to point" connection? second question is taking router A as an example does 192.168.0.5 / 255.255.255.252 considered the gateway address for all hosts attached to e0 (and ofcourse the same apply to router B)?
Thanks alot.
I think you need to study and understand much better what IP subnets ARE and WHY they exist, and how routers relate to these subnets. That is beyond the scope of answers in a forum like this ... read the manuals and learn! 
In brief, remember:
1) By definition, a router sits between subnets, moving traffic from one subnet to other subnets. You cannot (the router will not LET you) put two interfaces in the same subnet. This is why the serial link MUST be a different subnet from the ethernet LAN subnet, as must each ethernet interface if there is more than one being used on the router.
2) Hosts can only communicate with other devices in their subnet. If they have traffic that needs to leave the local subnet, they must send the traffic to a router interface in the subnet to which they belong.
From these two things, you can figure it out yourself: which IP address on your router should be configured as the gateway address on your PC's? The ethernet address, or the address for the serial interface?
Additional hint: If your ethernet network is 172.16.0.0 /16, your PC has an address like 172.16.0.7 /16. Your serial link is 192.168.0.5 /30. If you configure your PC with gateway address 192.168.0.5, what will happen to the packets? And if you configure your PC with gateway address 172.16.0.1, what will happen to the packets?
HTH,
doctorcisco | |
|
| Yes that helped clear things in my mind, I would like to thank you for both the advice, and the clarification. |
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