OSPF wildcard. What does the block size mean?

in this example, the wildcard .7 comes from a block size of 8. What does that mean?

Lab_A(config-router)#network 192.168.10.64 0.0.0.7 area 0

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networker050184

Any interface with an IP within that range will be included in the OSPF process.

mikeybinec

In ospf, the wildcard mask is like a mirror opposite of the subnet mask. So, the block size of 8 is basically 6 hosts, one broadcast address, and the network ID.. 255-248=7 that's where the wildcard mask number comes from. 255.255.255.248 is the opposite of 0.0.0.7

hope that helps

Deathmage

Best way to look at wild cards is this:

Write down 255.255.255.255 and then take your custom mask and subtract it, this equals your wildcard mask. So for instance if you have a /27 your custom mask would be 255.255.255.224. So 255.255.255.255 - 255.255.255.224 = 0.0.0.31. 0.0.0.31 would be your wildcard mask.

Another way to look at it, your power of 2's number minus 1 is also your wildcard mask. For instance a Class C address with a 2^6 power would be 64, which is also the block size for a 255.255.255.192. You just minus 1 from 64 and that's your wildcard mask, 0.0.0.63.

hope this helps.

TallDude7

I compare it to a photo. subnet mask is the photo and the wildcard is the negative of the photo. Add them both together and all octets must equal 255 (255.255.255.255). So you can always get the wildcard from the subnet with easy subtraction

Ede890

block size - 1

SurferdudeHB

Why do you have to subtract -1 on the last octect?

e.g.

192.168.10.49/29 = ospf 192.168.10.48 0.0.0.7 area 0

Also, on this example (from Todd Lamnle's book), why is he using wild card 0.0.255.255?

Router LA
Fa0/0 192.168.20.1/24
S0/0/1 172.16.10.6/30

OSPF= network 192.168.20.0 0.0.255 area 0
OSPF= network 172.16.0.0 0.0.255.255

stylezunknown

SurferdudeHB wrote: »

Why do you have to subtract -1 on the last octect?

e.g.

192.168.10.49/29 = ospf 192.168.10.48 0.0.0.7 area 0

Also, on this example (from Todd Lamnle's book), why is he using wild card 0.0.255.255?

Router LA
Fa0/0 192.168.20.1/24
S0/0/1 172.16.10.6/30

OSPF= network 192.168.20.0 0.0.255 area 0
OSPF= network 172.16.0.0 0.0.255.255

0.0.255.255 is the opposite of a classful mask.

networker050184

It's not classfull or classless, it's a simple bit matching mechanism. This isn't subnetting we are talking here.

Ede890

SurferdudeHB wrote: »

Why do you have to subtract -1 on the last octect?

e.g.

192.168.10.49/29 = ospf 192.168.10.48 0.0.0.7 area 0

Also, on this example (from Todd Lamnle's book), why is he using wild card 0.0.255.255?

Router LA
Fa0/0 192.168.20.1/24
S0/0/1 172.16.10.6/30

OSPF= network 192.168.20.0 0.0.255 area 0
OSPF= network 172.16.0.0 0.0.255.255

OSPF= network 192.168.20.0 0.0.255 area 0 is an incorrect wildcard you would type 192.168.0.0 0.0.255.255 area 0
the reason being is the wildcard is a range operator so think of it as of the number of IP address that you want to be included in your OSPF process ex. I want every ip address from the 192.168.x.x address scheme (X = 0-255) included in my process. this method is pretty much the more efficient way of including every network in your process. Also with the wild card of 0.0.0.7 think of it like this the block size is 8 right. so you write down the wildcard of 0.0.0.7 becuase you already have one number of your block size listed in your statement already which is 192.168.0.0. remember 0 is a counting number in computers! so counting from 0 to 7 equals 8. which is once again your block size this is the way i remember it if am wrong someone correct it don't wanna lead anyone astray.

volfkhat

SurferdudeHB wrote: »

Also, on this example (from Todd Lamnle's book), why is he using wild card 0.0.255.255?

Router LA
Fa0/0 192.168.20.1/24
S0/0/1 172.16.10.6/30

OSPF= network 192.168.20.0 0.0.255 area 0
OSPF= network 172.16.0.0 0.0.255.255

Lot of people have chimed in.
Normally, i would pass...... but i can definitely relate to your Q.

For me,
Wildcards are EASY.... just don't think too deeply about them.

1)
All you need to ask yourself is --> How do i Determine the Subnet Mask?.

That's it.
Once you know the Subnet Mask... then you Also know the Wildcard.

If i told you the Subnet Mask is
255.255.255.0
Then, that means the WildCard is
0.0.0.255

Now,
Just by giving this the 'eyeball' test... you can see that i Just Flipped the numbers.

ANd you know what?? you are correct!

All i did was subtract "255" from each octet. That gives me the Wildcard.

The ENd!

Try it out yourself... and then give yourself a Pat on the back :]

SurferdudeHB

volfkhat wrote: »

Lot of people have chimed in.
Normally, i would pass...... but i can definitely relate to your Q.

For me,
Wildcards are EASY.... just don't think too deeply about them.

1)
All you need to ask yourself is --> How do i Determine the Subnet Mask?.

That's it.
Once you know the Subnet Mask... then you Also know the Wildcard.

If i told you the Subnet Mask is
255.255.255.0
Then, that means the WildCard is
0.0.0.255

Now,
Just by giving this the 'eyeball' test... you can see that i Just Flipped the numbers.

ANd you know what?? you are correct!

All i did was subtract "255" from each octet. That gives me the Wildcard.

The ENd!

Try it out yourself... and then give yourself a Pat on the back :]

Thank you!