Wildcard mask

n3twrk0p

Hello everyone,

I'm having difficulties understanding wildcard mask, bellow I will explain what I know so far if anyone could correct me If I'm wrong.

Final result of all this is to advertised the following addresses using wildcard masks using EIGRP:

10.1.222.2/27

---

This address won't be advertised
10.1.2.2/25
10.1.12.2/30
10.1.23.2/30

masks>>128 193 224 240 248 252 254 255
bits>>>> 128 64 32 16 8 4 2 1
wcmasks 127 63 31 15 7 3 1 0

addresses
10.1.222.2/27

---

This address won't be advertised
10.1.2.2/25
10.1.12.2/30
10.1.23.2/30

Lets say I just want to advertised the following three subnets of the four listed

10.1.2.2
10.1.12.2
10.1.23.2

10.1.222.2/27 So I write them out in binary.

10.1.222.2 0.0.01111010.255
10.1.2.2 0.0.00000010.255
10.1.12.2 0.0.00001100.255
10.1.23.2 0.0.00011010.255

I think of wildcard mask of grouping address so when it comes to create ACL'S i don't write that many statements, now from this previous list of addresses in binary, I can see that we can only group subnets 12 & 23 since they are even until the 3rd bit, leaving us with a wildcard mask of 0.0.31.255

10.1.222.2 0.0.01111010.255
10.1.2.2 0.0.00000010.255
10.1.12.2 0.0.|000|01100.255
10.1.23.2 0.0.|000|11010.255

Now my huge confusion is when you write out this wildcard mask on the network statement for this subnets to be advertise don't subnet 2 also get advertised? since it has consecutive 0's to the 6th bit as well? or since it does not have any other address to pair it with, you have to write out a separate mask of 10.1.2.2 0.0.0.0 ?

I'm having some confusions when it comes to create wildcard masks for discontiguous networks, if someone could tell me how far I'm right and what I need to fully understand when it comes to create wildcard masks when you have a range of discontiguous addresses, will be appreciated.

Thank you

wave

If you are advertising discontinuous networks using EIGRP you're probably best to match them based on the exact address. e.g.

10.1.2.2 0.0.0.0
10.1.12.2 0.0.0.0
10.1.23.2 0.0.0.0

If they were contiguous you could group them:

10.1.1.2
10.1.2.2
10.1.3.2

match with 10.1.0.0 0.0.3.255

You should keep them as close together as possible. If you had these:

10.1.1.2
10.1.2.2
10.1.3.2

AND 10.1.20.1 ....then you would use:

10.1.0.0 0.0.3.255

and create a separate entry for 10.1.20.1

10.1.20.1 0.0.0.0

You shouldn't use a wild card mask that spans multiple subnets that you don't need to advertise. You might end up making a mistake.

wave

Remember that when you match an IP with a wild card mask in EIGRP you are only matching the interface IP address, you're NOT telling EIGRP how to advertise the subnet.

If you want to be very accurate you can match all of your interfaces with 0.0.0.0

Even if you have /24's /23's /14's - whatever - the networks will still be advertised correctly if you match with 0.0.0.0

n3twrk0p

otherwise I would let a whole bunch of subnets go through as well. I fully understand now, thank you very much guys