Inverted SN calculation with EIGRP and OSPF different?

Hello fellow CCNA'ers,

I've noticed something peculiar while correcting my CCNA answers:

The following networks are for the OSPF routing protocol

10.10.10.60/27 inverted SN 0.0.0.31

My calculation: 2^5=32 -1=31

10.10.10.0/30 inverted SN 0.0.0.3

My calculation= 2^2=4 -1= 3

With this method I calculate the last digit in my inverted SN.

Now for the part I do not understand:

The following networks are for the EIGRP routing protocol

192.168.10.4/30 =>same story as for the /30 with OSPF

192.168.10.128/26 inverted SN 0.0.0.192

My calculation: 2^6= 64 -1=63 => 0.0.0.63

Can someone please explain?

many thanks,

Jeroen[/b]

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Comments

loboernesto

192.168.10.128/26 inverted SN 0.0.0.192

Where did you get this wildcard?

a wildcard for a prefix /26 would be 63 as you correctly got it....

I think you mixed up a subnet mask 255.255.255.192 = /26
with the wildcard mask 0.0.0.63 am i right??

Jeroen1000

loboernesto wrote:

192.168.10.128/26 inverted SN 0.0.0.192

Where did you get this wildcard?

I downloaded an old CCNA3 final from the internet for practising purposes.
I guess Cisco dropped the ball a little with this question

Anyway, thank you for straightening this out, those SN things are quite confusing!

DirtySouth

Yah, I think you're getting Subnetmask & Wildcard mask mixed up. When configuring EIGRP, you need to specify subnet mask, however OSPF requires a wildcard mask.

loboernesto

When configuring EIGRP, you need to specify subnet mask, however OSPF requires a wildcard mask.

you are right, however you can now use wildcard as well as a common subnetmask card with eigrp

bmauro

I'll have to break open my book - but how can you use both? (Wildcard or subnet mask)

Like below:

Router(config)#router eigrp 100
Router(config-router)#network 172.16.1.0 255.255.255.0

Now using a subnet mask that would be a /24 mask - but as a wildcard it would imply something completely different.

How would the router know if you meant a subnet mask compared to a wildcard. (I might answer my own question once I find my book)

loboernesto

arriba(config)#router eigrp 1
arriba(config-router)#network 11.0.0.0 255.255.255.0
arriba(config-router)#net
arriba(config-router)#network 12.0.0.0 0.0.0.255
arriba(config-router)#end
arriba#show ip pro
*Mar 1 09:55:19.189: %SYS-5-CONFIG_I: Configured from console by console
arriba#show ip pro
Routing Protocol is "eigrp 1"
Outgoing update filter list for all interfaces is not set
Incoming update filter list for all interfaces is not set
Default networks flagged in outgoing updates
Default networks accepted from incoming updates
EIGRP metric weight K1=1, K2=0, K3=1, K4=0, K5=0
EIGRP maximum hopcount 100
EIGRP maximum metric variance 1
Redistributing: eigrp 1
EIGRP NSF-aware route hold timer is 240s
Automatic network summarization is in effect
Maximum path: 4
Routing for Networks:
11.0.0.0/24
12.0.0.0/24
Routing Information Sources:
Gateway Distance Last Update
Distance: internal 90 external 170

arriba#

I've just done this in my router. I think it has to do with the IOS because with an older IOS I had, mask information wasn't an option after network 10.0.0.0 command. But as the output shows the two configuration have the same effect.

cheers

ebykm

calculating wildcard masks are easy,

eg. if u have a subnet mask of 255.255.192.0, wildcard mask = 0.0.63.255. Subtracting the mask from 255 gives you the wildcard mask.

you could also specify a block of networks. just remember 0 in wildcard mask means exact match, others are don't care bits.