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

Question

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

Answer 1

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