VLSM

[carl_townshend]

Hi all, a quick question, If I have a /24 address given to me, can I subnet this down to different networks but still be able to use the whole /24 network as well ? or wouldnt this be advisable once you have subnetted it down ?

[Anand Narayana]

Hi Carl,

as mentioned in ur post, /24 meanz max. u can have 254 host, if u wanted to subnet,then

/30 - 2 hosts.(255.255.255.252)

/29 - 6 hosts. (255.255.255.248)

/28 - 14 hosts.(255.255.255.240)

/27 - 30 hosts.(255.255.255.224)

/26 - 62 hosts.(255.255.255.192)

/25 - 126 hosts.(255.255.255.128)

/23 - 510 hosts.(255.255.254.0)

/22 - 1022 hosts.(255.255.252.0)

& so on.

hope this helps.

rate this post.

[jackyoung]

What I think once you subnet the class then you can only use /24 for IP that does not be subnetted. Because there is more specific routes for those subnet and the remaining will fall into the /24 route (if there is).

Hope this helps.

[carl_townshend]

So once you have subnetted the class C are you saying you cant, or dont use the /24 ?

[gpulos]

once you subnet the class C, the only way you can use the /24 is as a 'summary-route'.

[carl_townshend]

thanks for that, I gather this is conventional subnetting, how about if I wanted to use vlsm ? i.e have my /24 into 2 /25 , 4 /26's etc

how would I achieve this ?

[jackyoung]

Just as Anand said, you can use x.x.x.0/25 & x.x.x.28/25 for 2 x /25 and x.x.x.0/26, x.x.x.64/26, x.x.x.128/26 & x.x.x.192/26 for 4 x /26 subnets. But please rememeber some of the routing protocol support non-VLSM (e.g. RIP).

Hope this helps.

[glen.grant]

you are correct that is how you would do it but rememeber if this is a current network with devices addressed as a /24 you will have to go out and readdress all the clients with addresses that fall within each subnet and change the subnet mask on all devices , if you are using dhcp it will make this easier...

[jackyoung]

Totally agreed and more make sense to use DHCP for eaiser management.