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Help with OSPF & wildcard masks
![--chris--](https://us.v-cdn.net/6030959/uploads/userpics/navatar350346_3.gif)
in CCNA & CCENT
I have been stumped by this scenario/question for the better part of today.
![2cxeo1e.jpg](http://i62.tinypic.com/2cxeo1e.jpg)
Can anyone help me out here? None of this is clicking for me.
![2cxeo1e.jpg](http://i62.tinypic.com/2cxeo1e.jpg)
Can anyone help me out here? None of this is clicking for me.
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prdemon Member Posts: 54 ■■■□□□□□□□
15-255 is 240 .....a 240 subnetmask is in increments of 16 ( 17-30 ) sorry if if didnt help any. -
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rocdamike Member Posts: 32 ■■□□□□□□□□
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 -
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gc8dc95 Member Posts: 206 ■■□□□□□□□□
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. -
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rocdamike Member Posts: 32 ■■□□□□□□□□
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. -
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OfWolfAndMan Member Posts: 923 ■■■■□□□□□□
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.:study:Reading: Lab Books, Ansible Documentation, Python Cookbook 2018 Goals: More Ansible/Python work for Automation, IPSpace Automation Course [X], Build Jenkins Framework for Network Automation [] -
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prdemon Member Posts: 54 ■■■□□□□□□□
Im sure this site has been posted before
Wildcard Mask Calculator -
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elderkai Member Posts: 279
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. -
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gc8dc95 Member Posts: 206 ■■□□□□□□□□
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 ^^^ -
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cpartin Member Posts: 84 ■■□□□□□□□□
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
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rocdamike Member Posts: 32 ■■□□□□□□□□
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! -
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--chris-- Member Posts: 1,518 ■■■■■□□□□□
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. -
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OfWolfAndMan Member Posts: 923 ■■■■□□□□□□
It's asking about e1/2. Not e1/1:study:Reading: Lab Books, Ansible Documentation, Python Cookbook 2018 Goals: More Ansible/Python work for Automation, IPSpace Automation Course [X], Build Jenkins Framework for Network Automation [] -
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atorven Member Posts: 319
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. -
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Heero Member Posts: 486
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. -
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MrPuzzlez Member Posts: 89 ■□□□□□□□□□
I would try it myself, but PT doesn't acknowledge the pipe command... -
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mikeybinec Member Posts: 484 ■■■□□□□□□□
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)Cisco NetAcad Cuyamaca College
A.S. LAN Management 2010 Grossmont College
B.S. I.T. Management 2013 National University -
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cpartin Member Posts: 84 ■■□□□□□□□□
I would try it myself, but PT doesn't acknowledge the pipe command...
Glad my demo helped. When in doubt, lab it out. -
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--chris-- Member Posts: 1,518 ■■■■■□□□□□
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. -
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--chris-- Member Posts: 1,518 ■■■■■□□□□□
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! -
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Gallager00 Member Posts: 29 ■□□□□□□□□□
Practice, Breathe, and eat subnetting all day evry'day2016 Goals: CCNA Security, CCNA Data Center, VCP6-NV. Mostly focusing on skills rather than certs.
2016 Completed:
Currently reading: CCNA Security, programming books