Meaning of this show IP route output in InterVLAN routing (subnet calculation) - did i get mistaken ?

[SJ K]

Hi all,

I am reading the configuration of interVLAN routing on 3750 from cisco @

http://www.cisco.com/c/en/us/support/docs/lan-switching/inter-vlan-routing/41260-189.html

There are 3 VLAN created on the L3 switch namely

VLAN10 - 10.1.10.0/24 network

VLAN 2 - 10.1.2.0/24 network

VLAN 3 - 10.1.3.0/24 network

But on the show IP route results (see bold red), why does it indicate that 10.0.0.0/24 is subnetted. How is it subnetted ?

10.1.10.0/24, 10.1.2.0/24, 10.1.3.0/24 all belongs to different network are not subnet out from 10.0.0.0/24.

How does the calculation goes ?

Cat3550#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
       D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
       N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
       E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
       i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area
       * - candidate default, U - per-user static route, o - ODR
       P - periodic downloaded static route

Gateway of last resort is 200.1.1.2 to network 0.0.0.0

     200.1.1.0/30 is subnetted, 1 subnets
C       200.1.1.0 is directly connected, FastEthernet0/48
     10.0.0.0/24 is subnetted, 3 subnets
C       10.1.10.0 is directly connected, Vlan10
C       10.1.3.0 is directly connected, Vlan3
C       10.1.2.0 is directly connected, Vlan2
S*   0.0.0.0/0 [1/0] via 200.1.1.2

Please advise

Regards,

Noob

[Reza Sharifi]

Hi,

The reason for that is because the route table has 3 subnets and they are all /24.  So the the router is letting you know that you have 3 /24 subnets. All subnets are /24s.

HTH

[Richard Burts]

To add a little to the information from Reza - the original poster is dealing with class A network 10.0.0.0 and that is where the line is red is talking about the major network 10.0.0.0 which has been subnetted. There are 3 subnets and they are consistent in their subnet mask which is /24. So the line in red is indicating that class A network 10.0.0.0 is subnetted with a /24 mask. 

Perhaps we need to clarify some terminology. The original post says this "10.1.10.0/24, 10.1.2.0/24, 10.1.3.0/24 all belongs to different network" which is technically incorrect. 10.1.10.0, 10.1.2.0, and 10.1.3.0 are all subnets derived from class A network 10.0.0.0.

HTH

Rick

[SJ K]

Hi Richard and Reza,

I thought 10.0.0.0/24 ->

NetworkId - 10.0.0.0

Range - 10.0.0.1 - 10.0.0.254

Broadcast - 10.0.0.255

=====================================

- How does 10.1.10.0/24, 10.1.2.0/24, and 10.1.3.0/24 get derived from 10.0.0.0/24 ?

Regards,
Noob

[Jon Marshall · Accepted solution]

It's really the difference between "network" and "subnet".

A lot of people use "subnet" everywhere including me most of the time but Rick is technically correct in that the network is 10.0.0.0/8 and from that network you have 3 subnets of that major network all of them using a /24 mask.

Because nowadays most of the time we are talking about classless IP addressing the terms have become almost interchangeable but there is a technical difference.

Jon

[SJ K]

Hi Jon,

Thanks for replying. I am also thinking along the line that it is 10.0.0.0/8 network whereby it is subnetted to the 24th bit, hence the 3 subnets are part of this major network.

But this bring back the question that

q1) what makes the router assume that it is a class A 10.0.0.0/8 network, just because of the leading bit which is "0" ? what if the network is actually maybe 10.0.0.0/10 which is subnetted further ? ( i guess it doesn't matter because the router is in fact showing 10.0.0.0/24 - meaning that it has subnetted to the 24th bit)

q2) does router still display routes as classful network ? what is the point of displaying the 10.0.0.0/24 as a general main line on top ?

Regards,
Noob

[Richard Burts · Accepted solution]

Noob

Jon is quite correct that in modern usage we tend to treat network and subnet as almost interchangeable. But technically there is a difference and that difference becomes significant for the kind of question that you are asking. There is no "network" 10.0.0.0/10. 10.0.0.0/10 is a subnet of the class A network 10.0.0.0/8. You are correct that 10.0.0.0/10 can be further subnetted but that does not make 10.0.0.0/10 into a "network".

To go a step further in explaining this perhaps we can think of designing a network for a company that has offices in several cities. We might assign 10.0.0.0/10 as the network for the Chicago office, and 10.64.0.0/10 as the network for the New York office, and 10.128.0.0/10 as the network for the Atlanta office and 10.192.0.0/10 as the network for the Los Angeles office. (Note that while I called them network here they are actually subnets of class A 10.0.0.0/8) Within each city we might further subnet their block of addresses to create multiple subnets for each city.

It might help to think about how Cisco organizes the routing table to support the routing function. When a router receives a packet and needs to make a forwarding decision it searches the routing table looking for the longest match. In functional terms what it is doing is to identify what network the packet belongs to and then to determine whether that network has been subnetted, and if so to which subnet does the packet go. So Cisco organizes the routing table to identify the network on one line and then to identify the subnets on lines below the network line. So in your original post the line in red

 10.0.0.0/24 is subnetted, 3 subnets

is telling us about the network and the lines below it are telling us about the subnets that it knows of that network.

It also seems that you are looking at 10.0.0.0/24 as if that were a single piece of information indicating that 10.0.0.0/24 is present in the routing table. That is not what is actually indicated. There are two separate and distinct pieces of information in that.

1) the network is 10.0.0.0 (a class A network)

2) the network is subnetted consistently using a /24 mask

HTH

Rick

[SJ K]

Hi Rick,

Thanks. Wonderful explanation.  I am straight now. ;)

Regards,
Noob

[Richard Burts]

Noob

I am glad that our answers were helpful and that you are straight about this aspect of networking now. Thank you for using the rating system to mark answers as correct. This helps other readers of the forum to identify threads that have helpful information.

HTH

Rick

[SJ K]

Hi Rick,

No problem. I should be thanking you and the folks here instead in the forum that is assisting  us rookies all the time.

Thank you.

Regards,
Noob