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TOPIC: Confused on ip default-network

Confused on ip default-network 9 years 7 months ago #31665

Guys, I tried searching here but i cant find a topic that clearly explain about ip default-network command. been reading some topics about default-network. But until now, I still dont get the concept behind..

in this picture, you can see the R1 has made a command for ip default-networkm, which means it becomes the gateway of last resort to network

in here, when we check on R3's routing table, we can see that the gateway of last resort is R1 to network

so a ping confirms that it can connect outside.
means that when a packets destined for another subnet it uses the link to R1 to reach

But a ping to fails!?! how come ?!?! ~__~

so the book says, that if you configure an ip classless command
itll eventually make a successful ping..

now my question is, if ping can be reached using R3, using the default route of R1 to connect outside.. then how come a ping to fails which has a default route to R1 also.
but after configuring an ip classless the ping works..

i dont really get the logic behind it..

Re: Confused on ip default-network 9 years 7 months ago #31679

What about layer 2 in this case? If you are running "no ip classless" it does not impact the routing table but does impact routing decisions when using VLSM. The routing decision can only account for (this is how is being accounted) because of "no ip classless". Every interface on R3 is in the major net. The decision for ethernet is to ARP for this address or inverse arp on your frame-relay serial link. There is no routing if it "thinks" the network is directly connected. It can route via the default route to the address because there is no directly connected 10 network on the router. If you put a loopback interface on R3 of with "no ip classless" can you still reach via R1?

Enter "ip classless" and now routing decisions can be made with VLSM, and it can account for all the networks as they show in the table. It can see past the major net block.


Re: Confused on ip default-network 9 years 7 months ago #31684

are you saying that the routing protocol being used doesnt know about VLSM?

im sorry but i dont get your point.. im very slow.. >,<

default network 9 years 7 months ago #31686

It seems that, the pic you posted is from Cisco press book ICND-2, and for complete network diag see page no.186, under topic static route

From your given conf. it is clear that R3 learns a default route from R1 using RIP, R3 lists the destination of the default route ( and the next-hop router, which is R1 in this case (, as highlighted in your pic.

In your 2nd pic 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, (In R3 show Ip route command showing network i.e. found in R3 routing table) 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 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 command.

after confuguring IP classless command, In classless routing Cisco IOS Software looks for the best match, ignoring class rules. If a default route exists, with classless routing, the packet always at least matches the default route. If a more specific
route matches the packet’s destination, that route is used.

I hope so this explanation will help you to clear your concept.



Re: Confused on ip default-network 9 years 7 months ago #31687

In your 2nd pic R3 is configured to use classful routing.

how can you tell if the router is using a classful? is it because its using RIP? or because the ip default-network is configured?

Default Network. 9 years 7 months ago #31720

Dear, In your 2nd pic it is absolutely showing that, R3 configured with classful routing.

see the sentence :- is subnetted, 4 subnets

Also network diag. showing that entire network is a classful network.

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