Answer to Monday’s Subnetting Speed Test Question

Monday’s question was about interpreting the output of two show commands, show ip route and show ip interface brief. Answers are below the fold. Enjoy!

Answers: A, F

The key to answering this question is to remember the format and content of the output in the commands listed in the question. If you didn’t recall those details, refer here to Example 2-1, which lists the output.

R2#show ip interface brief 
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0/0        172.16.2.2      YES manual up                    up
FastEthernet0/1        unassigned      YES unset  administratively down down
Serial0/0/0            10.1.23.2       YES manual up                    up
Serial0/0/1            10.1.12.2       YES manual up                    up
R2#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 not set
10.0.0.0 255.255.255.0 is subnetted, 2 subnets
C       10.1.12.0 is directly connected, Serial0/0/1
C       10.1.23.0 is directly connected, Serial0/0/0
172.16.0.0 255.255.255.128 is subnetted, 1 subnets

The lines that begin with “C”, as per the legend at the top of the output from the show ip route command, list all connected routes. Routers put connected routes in their own IP routing table when an interface has an IP address/mask assigned, and the interface is currently in an up/up state.

IOS lists the masks associated with each route either in a heading line for the entire classful network, or per-line if the mask differs for subnets in the same network (in other words, when Variable Length Subnet Masks, VLSM, is used). In this case, with all known subnets of network 10.0.0.0 using mask /24, the mask is on the heading line.

If the user issues the terminal ip netmask-format decimal command, IOS lists the mask in prefix notation, so in this case, the output of show ip route will list masks in dotted decimal notation (DDN).

The show ip interface brief command does not list the masks at all. Instead, it lists the IP address associated with each interface, as well as interface status. It does not list the subnet number of the connected subnets, but rather the IP address as configured with the ip address interface subcommand.

For the three answers that list show ip interface brief, only one answer (A) lists an IP address of an interface on router R2, so answer A is correct. In particular, note that answer E lists a prefix style mask, and the show ip interface brief command does not list masks at all.

For the three answers that mention the show ip route command, note that according to the question text, this command occurs after the user has told IOS to list masks in decimal notation. As a result, answer F is correct, because R2’s F0/0 interface has been configured with mask /25 (255.255.255.128), as implied in the figure for this question set.

The other two answers that mention the show ip route command (B and D) both list DDN numbers, but one (10.1.12.1) is an interface IP address instead of the subnet connected to an interface, so R2 does not list 10.1.12.1. The other incorrect answer (D) lists a subnet number (172.30.4.0), but it exists off router R4. According to the question, the configuration so far does not include any routing protocol configuration, nor any static routes, so R2 would not yet know a route to reach this subnet.