Configure OSPF on a port, why when entering ip address of port it's minus 1

I'm reading Todd Lammle's ICDN1 and on his example for configuring (using wildcards) on a router, the IP address is -1 on the last octect. See below..

Router Lab B
G0/0: 10.255.255.82/30
G0/0: 10.255.255.9/30
Fa0/0: 192.168.10.49/29

router configuration:
#config t
#router ospf 1
#network 10.255.255.80 0.0.0.3 area 0
#network 10.255.25.8 0.0.0.3 area 0
#network 192.168.10.48 0.0.0.7 area 0

Can someone explain this to me, thx.

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networker050184

That is using the interface network. Just like if you created a static route you'd route the network 10.255.255.80 255.255.255.252.

10.255.255.80/30

network 10.255.255.80
first usable 10.255.255.81
second usable 10.255.255.82
broadcast 10.255.255.83

You could just as easily use 10.255.255.82 0.0.0.0 area 0 though.

james43026

The best advice you can be given on this. Is to write this all out in binary, and look at it from there. When it comes to subnetting / subnet masks / wildcard masks, it's truly the best way to come to a true understanding of it all.

Nans

SurferdudeHB wrote: »

I'm reading Todd Lammle's ICDN1 and on his example for configuring (using wildcards) on a router, the IP address is -1 on the last octect. See below..

Router Lab B
G0/0: 10.255.255.82/30
G0/0: 10.255.255.9/30
Fa0/0: 192.168.10.49/29

router configuration:
#config t
#router ospf 1
#network 10.255.255.80 0.0.0.3 area 0
#network 10.255.25.8 0.0.0.3 area 0
#network 192.168.10.48 0.0.0.7 area 0

Can someone explain this to me, thx.

Buddy what he configured OSPF is with the network. So teh protocols will look for the interfaces in that network and send LSA's on those interfaces only. Since he always uses 1st address of the network as an interface address on router u might have assumed that it's always -1 the ip address on the routers interface.

So basically what he did there was he configured 10.255.255.80 0.0.0.3 area 0 so that the routing protocol will look for the interfaces in that network and thinks that the users was referring to this and snd's LSA's through that interface to establish adjacencies.

Hope this helps please correct me if I am wrong.

phantasm

james43026 wrote: »

The best advice you can be given on this. Is to write this all out in binary, and look at it from there. When it comes to subnetting / subnet masks / wildcard masks, it's truly the best way to come to a true understanding of it all.

Correct. Also, you'll need to be quite fast at subnetting when you take the exams.

As has already been stated. The book is using the wildcard mask to represent the NETWORK. So if your IP address is 10.1.2.1 with a network mask of 255.255.255.128 (which is a /25) then the wildcard mask is always the block size minus 1. In this case the wildcard mask would be 0.0.0.127.

SurferdudeHB

I got it! thanks ya'll. It's the network that is being configured!

One question on the wild card, why is it always the block size minus 1?

d4nz1g

Network statement: Matches every interfaces on a given range.
Ex:
F0/1 - 10.1.1.1/24
F0/2 - 10.2.2.1/30
F0/3 - 10.254.2.1/23

If you issue a network 10.0.0.0 0.255.255.255 area x under router ospf, it will enable ospf for every single interface stated above.

Wildcard mask: Same logic as ACL. Forget this -1 rule. It does not exist. It is just the network mask written backwards.

Example: 255.255.255.128 network mask will correspond to 0.0.0.127
Why? Turn all 1's to 0's and all 0's to 1's.

Shortcut: subtract the mask from 255.255.255.255

Example: what is the wildcard mask for 255.255.255.248?

Well, 255.255.255.255 minus 255.255.255.248 = 0.0.0.7.

networker050184

SurferdudeHB wrote: »

I got it! thanks ya'll. It's the network that is being configured!

One question on the wild card, why is it always the block size minus 1?

I think it's your way of going about it that is getting you caught up. Like before, write it out in binary to see the why. Learn the bits behind it before trying to spot patterns in dotted decimal form.