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New Member

useable addresses

can you use addresses with a 255 in them ? on my internet router it has and ip of x.x.255.x !! I thought you can only use up to 254 !!

thanks

  • Getting Started with LANs
5 REPLIES
Cisco Employee

Re: useable addresses

You can use an IP address with 255 in it, as long as its not the first or last octet

New Member

Re: useable addresses

why is this ? I thought that you can only have 254

Cisco Employee

Re: useable addresses

you can even use a 0 or 254 for the last octet depending on your mask

ex) 10.0.1.0/22

However, its better to stay away from 0 or 255

Green

Re: useable addresses

Here's the (very) short version:

If the host portion of the addressis all zeros, it's considered (by convention) to be the "Network Address" or "Segment Address".

If the host portion is all ones, it's considered, by convention, to be the Broadcast Address.

So, if you have an address like 10.1.254.255/16, it is a valid host address (though, not necessarily a smart choice) because the least significant bit of the third octet is a zero - hence, all of the host bits are not all zeros, or all ones.

You could also have an address like 10.254.255.255/7 and still have a valid (but not smart) host address.

You could have something like 10.1.0.0/8 and still have a valid host address ... all of the host bits are not all zeros or all ones.

When we say it's not necessarily "smart" is because someone else may see those addresses, decide the person that set 'em up was an idiot, and change them to something .... or worse, get confused & mess up on the subnet mask and kill a bunch of nets or subnets (and dynamic routing ....).

OK, so it wasn't all that short ...

FWIW

Scott

New Member

Re: useable addresses

Hello

check this link

http://www.cisco.com/warp/public/701/3.html

you will have answer to your question.

HTH

cisand

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