Help with OSPF & wildcard masks

--chris--

I have been stumped by this scenario/question for the better part of today.

Can anyone help me out here? None of this is clicking for me.

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Comments

prdemon

15-255 is 240 .....a 240 subnetmask is in increments of 16 ( 17-30 ) sorry if if didnt help any.

rocdamike

If it makes you feel any better, I can't get my head around this either. My own personal answer would be: network 192.168.100.16 0.0.0.15 area 0, but this is not an available option. Hopefully someone else will be able to clear this up

gc8dc95

If you only want e1/2 to be enabled for ospf then you would need 255.255.255.240 = 192.168.100.16 - 192.168.100.31

192.168.100.8 /29 is a network address.

rocdamike

gc8dc95 wrote: »

If you only want e1/2 to be enabled for ospf then you would need 255.255.255.240 = 192.168.100.16 - 192.168.100.31

192.168.100.8 /29 is a network address.

I agree in that the answer is not A. But why is the answer stated as B? I don't understand how 192.168.100.30 is correct as it's not a network address.

OfWolfAndMan

The correct answer is d. Your SIM is wrong.

Just think of it this way: 0s are a 255. Take the last number where it's not 0. Add 1 to it. That's your increment. The problem with .30 is it will include the E1/3 interface.

prdemon

Im sure this site has been posted before
Wildcard Mask Calculator

elderkai

The SIM isn't wrong. .30 is within the 192.168.100.16/28 network, so it'll be enabled on any interface with IP address inside that network.

gc8dc95

elderkai wrote: »

The SIM isn't wrong. .30 is within the 192.168.100.16/28 network, so it'll be enabled on any interface with IP address inside that network.

This ^^^

cpartin

rocdamike wrote: »

I agree in that the answer is not A. But why is the answer stated as B? I don't understand how 192.168.100.30 is correct as it's not a network address.

IOS will auto-correct your statement, you don't have to put the network address in the command.

R1(config)#router ospf 1
R1(config-router)#network 192.168.100.30 0.0.0.15 area 0
R1(config-router)#^Z
R1#show run | section ospf
router ospf 1
 log-adjacency-changes
 network 192.168.100.16 0.0.0.15 area 0
R1#
R1#show ip ospf interface
Serial1/2 is administratively down, line protocol is down
  Internet Address 192.168.100.18/31, Area 0

rocdamike

cpartin wrote: »

IOS will auto-correct your statement, you don't have to put the network address in the command.

R1(config)#router ospf 1
R1(config-router)#network 192.168.100.30 0.0.0.15 area 0
R1(config-router)#^Z
R1#show run | section ospf
router ospf 1
 log-adjacency-changes
 network 192.168.100.16 0.0.0.15 area 0
R1#
R1#show ip ospf interface
Serial1/2 is administratively down, line protocol is down
  Internet Address 192.168.100.18/31, Area 0

Now it has finally clicked. I didn't know you could do that. Thank you ever so much for showing the config. I can get a restful night's sleep now!

--chris--

elderkai wrote: »

The SIM isn't wrong. .30 is within the 192.168.100.16/28 network, so it'll be enabled on any interface with IP address inside that network.

But what about the 192.168.100.12? Its not in that network. Thats where I get lost. I can see how we arrive at the 240 mask, but I dont see how this would then bring the .12 into OSPF.

OfWolfAndMan

It's asking about e1/2. Not e1/1

atorven

I should learn to read, I was reading that as Ethernet 1/1 and Ethernet 1/2. B is the correct answer as 18 falls in between 16-31.

Heero

The wildcard mask tells the router which bits of the network address to ignore when matching to an interface. 0.0.0.15 tells the router to ignore the last 4 bits of the network address.

192.168.100.30 = 11000000 10101000 01100100 00011110

If you ignore the last 4 bits, you get 11000000 10101000 01100100 0001XXXX, which = 192.168.100.16-192.168.100.31.

That will match eth1/2. In practice, you won't ever really see it configured that way on the router's ospf process. It is more of a trick question to make sure you understand how the network statement works and how the wildcard mask works.

MrPuzzlez

I would try it myself, but PT doesn't acknowledge the pipe command...

mikeybinec

YOU HAVE A BOSON SIM!!!!

B is the correct answer. because the .30 falls into the block size of 16.

Surprised OfWolfandMen is still sticking to D.. (.15 is a broadcast address)

cpartin

MrPuzzlez wrote: »

I would try it myself, but PT doesn't acknowledge the pipe command...

You could always just do a full "show run", I was just getting the relevant bits. The ability to pipe and filter output is just ridiculously useful... that really sucks PT doesn't support it.

Glad my demo helped. When in doubt, lab it out.

--chris--

atorven wrote: »

I should learn to read, I was reading that as Ethernet 1/1 and Ethernet 1/2. B is the correct answer as 18 falls in between 16-31.

Jesus....this is the same thing I was doing. I was reading that it wanted two interfaces added into OSPF.

Well now that I know how to do it, I should learn to read the entire question and ensure I understand it.

--chris--

mikeybinec wrote: »

YOU HAVE A BOSON SIM!!!!

B is the correct answer. because the .30 falls into the block size of 16.

Surprised OfWolfandMen is still sticking to D.. (.15 is a broadcast address)

Yes, and its awesome. The explanations, charts of weak areas and sims have been great. Highly recommended for anyone on the fence. I have learned a lot from the sims explanations!

Gallager00

Practice, Breathe, and eat subnetting all day evry'day