General technical discussions.
mitm
Junior Member
Posts:
62
Joined:
Tue Feb 07, 2012 4:13 pm
Certs:
CCENT, CCNA

Class A, B, or C network

Sat Apr 21, 2012 2:51 pm

Hey guys,

What is the Class A, B, or C network of the following address:

168.13.2.0 /24

Thanks....

User avatar
mellowd
CCIE #38070
Posts:
13814
Joined:
Wed Jun 18, 2008 7:49 am
Certs:
CCIE (RS,SP), JNCIE-SP, BC-/SPNE/NP

Re: Class A, B, or C network

Sat Apr 21, 2012 3:11 pm


mitm
Junior Member
Posts:
62
Joined:
Tue Feb 07, 2012 4:13 pm
Certs:
CCENT, CCNA

Re: Class A, B, or C network

Sat Apr 21, 2012 6:56 pm

mellowd wrote:http://en.wikipedia.org/wiki/Classful_network


So....the Classful B network is 168.13.0.0, right?

So....here's my problem. If a router is using classful routing with the no ip classless command and it has the following routing table:

Default Route.jpg
Default Route
Default Route.jpg (147.6 KiB) Viewed 344 times


Why did the ping 168.200.1.1 fail?

ICND2 by Odom says,
R3 is configured to use classful routing. With classful routing, the router first matches the Class A, B, or C network number in which a destination resides. If the Class A, B, or C network is found, Cisco IOS Software then looks for the specific subnet number. If it isn't found, the packet is discarded, as is the case with the ICMP echoes sent with the ping 168.200.1.1 command. However, with classful routing, if the packet does not match a Class A, B, or C network in the routing table, and a default route exists, the default route is indeed used--which is why R3 can forward the ICMP echoes sent by the successful ping 10.1.1.1 command.

In short, with classful routing, the only time the default route is used is when the router does not know about any subnets of the packet's destination Class A, B, or C network.


So, if the destination address is 168.200.1.1, the classful B network is 168.200.0.0, correct? Since there aren't any subnets of this network in the routing table, shouldn't the ping 168.200.1.1 succeed by using the default route?

Thanks.....

mitm
Junior Member
Posts:
62
Joined:
Tue Feb 07, 2012 4:13 pm
Certs:
CCENT, CCNA

Re: Class A, B, or C network

Sat Apr 21, 2012 10:59 pm

So, if the destination address is 168.200.1.1, the classful B network is 168.200.0.0, correct? Since there aren't any subnets of this network in the routing table, shouldn't the ping 168.200.1.1 succeed by using the default route?


Oh, wait.....this is wrong. I mean if the destination address is 168.200.1.1, the classful B network is 168.200.0.0. Therefore, the classful B network is not in the routing table so the ping 168.200.1.1 should succeed by using the default route. Is this correct?

User avatar
mellowd
CCIE #38070
Posts:
13814
Joined:
Wed Jun 18, 2008 7:49 am
Certs:
CCIE (RS,SP), JNCIE-SP, BC-/SPNE/NP

Re: Class A, B, or C network

Sun Apr 22, 2012 1:30 am

It should be using the default route yes, but what about return traffic? What does the route table look like on the responder? What happens when you traceroute? What happens when you ping with a source address of your Ethernet interface?

mitm
Junior Member
Posts:
62
Joined:
Tue Feb 07, 2012 4:13 pm
Certs:
CCENT, CCNA

Re: Class A, B, or C network

Sun Apr 22, 2012 11:39 am

mellowd wrote:It should be using the default route yes, but what about return traffic? What does the route table look like on the responder? What happens when you traceroute? What happens when you ping with a source address of your Ethernet interface?


Hey mellowd,

This isn't a lab exercise. This is written material from the ICND2 book by Odom regarding the use of the default route when using classful or classless routing. I'm guessing all routes exist for communication, but the use of the default route and, therefore, successful communication depends upon ip classless/no ip classless configuration.

According to Odom the ping 168.200.1.1 should fail because no ip classless is configured, the Classful B network (???) of the destination is in the routing table, and the default route is not used. So, the router, R3, is discarding the ICMP Echo Request.

I, on the other hand, believe the no ip classless is configured, Classful B network 168.200.0.0 is not in the routing table, the default route is used, and, therefore, the ping succeeds. I think the book is wrong. Don't you agree?

mitm

User avatar
mellowd
CCIE #38070
Posts:
13814
Joined:
Wed Jun 18, 2008 7:49 am
Certs:
CCIE (RS,SP), JNCIE-SP, BC-/SPNE/NP

Re: Class A, B, or C network

Sun Apr 22, 2012 11:47 am

Unfortunately difficult to know exactly without having the book here :(

'

Return to General Tech

Who is online

Users browsing this forum: Yahoo [Bot] and 13 guests