 MattAll noise, no signal.Premium
join:2003-07-20
Jamestown, NC
kudos:12 | Break /25 into small and large subnet? Hello Folks,
Is it possible to take a /25 subnet and break it up even further, preferably into a /30 and then whatever the largest possible from the remaining addresses?
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"What is conservatism? Is it not adherence to the old and tried, against the new and untried?" - Abraham Lincoln |
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 DaSneaky1Done wall to block them allPremium,MVM
join:2001-03-29
The Lou | Yes. You have 128 IP's to subnet into. |
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 tubbynetreminds me of the danse russePremium,MVM
join:2008-01-16
Chandler, AZ
1 edit | reply to Matt
said by Matt:Is it possible to take a /25 subnet and break it up even further, preferably into a /30 and then whatever the largest possible from the remaining addresses? yes. you need to make sure that your gears support classless addressing and variable-length subnet masks (vlsm). most new gears (network-centric) support this, though if you're running anything that is compiled (say, a *nix box somewhere), you'll need to look at the appropriate man pages, doc files, subversion changelogs, forum posts, or tea leaves.
that being said, remember that every time you break apart a subnet into futher subnets, you lose an additional two ip's (resulting from the new networks "network" and "broadcast" address). not a *huge* deal, but if you're planning on expanding, this may become an issue.
q.
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"...if I in my north room dance naked, grotesquely before my mirror waving my shirt round my head and singing softly to myself..." |
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Bink
join:2006-05-14
Denver, CO
kudos:4 | FWIW, I’ve yet to find a UNIX box that didn’t do classless addressing. Which 25+ year old UNIX machine were you using that didn’t support classless addresses? |
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tubbynetreminds me of the danse russePremium,MVM
join:2008-01-16
Chandler, AZ | said by Bink:FWIW, I’ve yet to find a UNIX box that didn’t do classless addressing. Which 25+ year old UNIX machine were you using that didn’t support classless addresses? i haven't ever done anything classless on a *nix box. my comments are only valid for cisco and juniper gears (though sonicwall, force10, and the like all support it) since that is what i know. it was more of a disclaimer than anything, especially since i know that Matt  has talked about using *nix in the past. i am not nearly as versed in *nix as you are, and you have answered the remaining doubt 
my comment regarding *nix was for Matt to verify that what he was using did support it (though it was a very feeble attempt to make a funny with places to check).
q.
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"...if I in my north room dance naked, grotesquely before my mirror waving my shirt round my head and singing softly to myself..." |
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Bink
join:2006-05-14
Denver, CO
kudos:4 | Well, both Cisco’s and Juniper’s roots lie in BSD—which had the best TCP/IP stack at the time (even Microsoft used it)—so stop hating on UNIX  . |
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MattAll noise, no signal.Premium
join:2003-07-20
Jamestown, NC
kudos:12 | reply to Matt
If I use VLSM and only want one /30, won't I have to break the entire subnet up into 64 individual subnets? |
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tubbynetreminds me of the danse russePremium,MVM
join:2008-01-16
Chandler, AZ | said by Matt:If I use VLSM and only want one /30, won't I have to break the entire subnet up into 64 individual subnets? no, the purpose of vlsm support is that a single device (router, layer3 switch) can support different subnet masks.
q.
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"...if I in my north room dance naked, grotesquely before my mirror waving my shirt round my head and singing softly to myself..." |
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MattAll noise, no signal.Premium
join:2003-07-20
Jamestown, NC
kudos:12
1 edit | said by tubbynet:said by Matt:If I use VLSM and only want one /30, won't I have to break the entire subnet up into 64 individual subnets? no, the purpose of vlsm support is that a single device (router, layer3 switch) can support different subnet masks. q. The how-to's I'm reading on VLSM are way off base then. They keep talking about sub-subnetting the subnet. Like breaking the /25 into 2 x /26, then halve those and so on ...
It's possible I'm approaching this wrong. I want to take my /25 and break it into two subnets. One that only needs one usable IP and then a big pool of addresses using what's left over.
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"What is conservatism? Is it not adherence to the old and tried, against the new and untried?" - Abraham Lincoln |
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tubbynetreminds me of the danse russePremium,MVM
join:2008-01-16
Chandler, AZ | said by Matt:The how-to's I'm reading on VLSM are way off base then. They keep talking about sub-subnetting the subnet. Like breaking the /25 into 2 x /26, then halve those and so on ... you don't need to keep breaking stuff up. i've routinely used /30's for point-to-point links (such as wireless radios, private t1s, etc) and then brought the other end of that link up on a different subnet, but still in the same address-space as defined by the standard "classful" boundaries.
what device are you looking at putting this on?
It's possible I'm approaching this wrong. I want to take my /25 and break it into two subnets. One that only needs one usable IP and then a big pool of addresses using what's left over.
thats exactly what you use vlsm, and cidr for
q.
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"...if I in my north room dance naked, grotesquely before my mirror waving my shirt round my head and singing softly to myself..." |
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aryobaPremium,MVM
join:2002-08-22
kudos:1 | reply to Matt
said by Matt:said by tubbynet:said by Matt:If I use VLSM and only want one /30, won't I have to break the entire subnet up into 64 individual subnets? no, the purpose of vlsm support is that a single device (router, layer3 switch) can support different subnet masks. q. The how-to's I'm reading on VLSM are way off base then. They keep talking about sub-subnetting the subnet. Like breaking the /25 into 2 x /26, then halve those and so on ... It's possible I'm approaching this wrong. I want to take my /25 and break it into two subnets. One that only needs one usable IP and then a big pool of addresses using what's left over. Depending on your needs, you can take first half of /26 off /25 as the single big subnet. The second half of /26 can be broken down into smaller subnets such as one /27, one /28, one /29, and two /30 subnets. This way you have total of 6 subnets off the /25 consists of one /26, one /27, one /28, one /29, and two /30 subnets. |
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