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Wildcard or subnet mask

chargenchargen Member Posts: 12 ■□□□□□□□□□
in CCNP
Did some searching, but haven't seen a nice easy method of memorizing when a subnet mask vs a wildcard mask is required. Is there a good rule of thumb, or does it just require memorization?

Thanks!

-Pete
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    networker050184networker050184 Mod Posts: 11,962 Mod
    Just memorize it. After you configure things a few times it sticks.
    An expert is a man who has made all the mistakes which can be made.
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    chargenchargen Member Posts: 12 ■□□□□□□□□□
    Cool, thanks
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    DevilWAHDevilWAH Member Posts: 2,997 ■■■■■■■■□□
    hit the ? mark button ;)

    I find in general when assigning applying a ip / range you use a subnet mask. when your are defining an range for filter/routing you use a wild card mask.

    but that is in no way 100% true
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
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    peanutnogginpeanutnoggin Member Posts: 1,096 ■■■□□□□□□□
    networker050184 wrote: »
    Just memorize it. After you configure things a few times it sticks.

    +1

    If you get some hands on configuration... it'll become one of those 2nd nature things that you don't really realize that you know it.

    -Peanut
    We cannot have a superior democracy with an inferior education system!

    -Mayor Cory Booker
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    johnwest43johnwest43 Member Posts: 294
    most of the time interfaces & dhcp settings = mask, everything else is wild-card. Definitely not an absolute though.

    Another goofy thing to remember is routers use wild-cards for acls and cisco pix/asa firewalls use masks.
    CCNP: ROUTE B][COLOR=#ff0000]x[/COLOR][/B , SWITCH B][COLOR=#ff0000]x[/COLOR][/B, TSHOOT [X ] Completed on 2/18/2014
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    DevilWAHDevilWAH Member Posts: 2,997 ■■■■■■■■□□
    Another thing to rember is that

    255.0.0.255 could be a valid wild card mask which would mean the middel octecs must match but the first and last could be anything.

    on the other hand a subnet mask must be split network bits / host bits
    255.248.0.0

    Wildcard masks are not reverse subnet (althogh 99% of the time we use revese subnet masts becuase we want to filter a subnet block of addresses)

    So if you are declaring what subnet and IP should be in, then you need a subnet mask, for filtering like ACL's its a wild card mask.

    Once ou have lernt a few diffent places you use each you will find there is a patten and logic to wheer you whould use each.
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
    • An arrow can only be shot by pulling it backward. So when life is dragging you back with difficulties. It means that its going to launch you into something great. So just focus and keep aiming.
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    peanutnogginpeanutnoggin Member Posts: 1,096 ■■■□□□□□□□
    DevilWAH wrote: »
    Another thing to rember is that

    255.0.0.255 could be a valid wild card mask which would mean the middel octecs must match but the first and last could be anything.

    Very interesting... I've never tried that. I'll have to lab it up. I couldn't see it as being too useful for me at the moment, but that would be a "show off" type of command on an ACL. icon_thumright.gif

    Thanks.

    -Peanut
    We cannot have a superior democracy with an inferior education system!

    -Mayor Cory Booker
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    DevilWAHDevilWAH Member Posts: 2,997 ■■■■■■■■□□
    peanutnoggin wrote: »
    Very interesting... I've never tried that. I'll have to lab it up. I couldn't see it as being too useful for me at the moment, but that would be a "show off" type of command on an ACL. icon_thumright.gif

    Thanks.

    -Peanut


    I have seen it used :) and it can be quite usefull,

    Imagen you have the following subnets below, and you want a filter that seperates between the x.x.10.x and x.x.20.x ranges

    usign subnet masks you whould have to have four statments. but using a wild card you could do it with one.

    address 172.16.10.0 wildcard 0.3.0.255, 1st octet match172 , second octet is 16 to 19. third octct match 10, and last octet can be any thing. And there you are, your mask is pulling out only the x.x.10.x networks from the list.

    so if you know how to use them they can be much more flexable and ecnomical than a subnet mask.

    172.16.10.x
    172.16.20.x
    172.17.10.x
    172.17.20.x
    172.18.10.x
    172.18.20.x
    172.19.10.x
    172.19.20.x
    • If you can't explain it simply, you don't understand it well enough. Albert Einstein
    • An arrow can only be shot by pulling it backward. So when life is dragging you back with difficulties. It means that its going to launch you into something great. So just focus and keep aiming.
    Linkin Profile - Blog: http://Devilwah.com
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