useable IP Address

[ex34i]

These question came ip in my networking class and my teacher was unsure how to answer it: How many IP address are there? My answer was 4,228,250,625 IP address. Then another question was asked, how many useable IP address are there? I know that a.b.c.255 is used for broadcasting, 127.b.c.d is used for loopbacks, and I also know that a.b.c.1 (or is it 0??) is used for something to but I don't know what. Is there anything else I am missing? Also one more question, why are the IP address 10.b.c.d and 172.b.c.d used more frequantly company networks?

 

[manarth]

10.x.x.x, 172.16.x.x-172.32.x.x., and 192.168.x.x are all private IPs, reserved for use in LANs behind a NAT router.

These addresses are unroutable and should never appear on the internet. Hence their use in corporate LANs.

See IANA for more about IP assignations.

<marc> i wonder what will happen if i press this...[ul][li]please tell us if our suggestion has helped[/li][li]need some help? faq581-3339[/li][/ul]

 

[Andyleates]

Class B is actually 172.16 through 172.31

Andy Leates MCSE CCNA MCP+I

 

[manarth]

failing memory - oops! I stand corrected, class B is indeed 172.16 -> 172.31

<marc> i wonder what will happen if i press this...[ul][li]please tell us if our suggestion has helped[/li][li]need some help? faq581-3339[/li][/ul]

 

[Chr1sUK]

the x.x.x.0 is the network address and cannot be assigned to a host.

 

[itsp1965]

Manarth also, just because they are private addresses does not mean they are non-routable. You can use private addressing in Wide Area Networks, although you are corect in your statement that they are not used on the Internet.

 

[manarth]

for my statement 'unroutable' read 'unroutable on the internet' - because all internet routers are (should be) configured to fail these IPs...

And although these IPs are not 'unroutable' in the strict sense, in practice it's only the big-boy routers which will route these IPs. Most SoHo routers will not - to ensure that even when the router's misconfigured, these IPs don't appear on the internet.

<marc> i wonder what will happen if i press this...[ul][li]please tell us if our suggestion has helped[/li][li]need some help? faq581-3339[/li][/ul]

 

[bierhunter]

To answer how many usable IP addresses are there is a tricky one. I think the question is too vague and leaves a lot open to interpretation.

For example: x.x.x.0 may be a network address for a /24, but could a host address in a /23 subnet depending upon where the subnet starts. Same goes for x.x.x.255 /23. That could actually be assigned to a host computer instead of being a broadcast as in a /24. Also, a network or broadcast address is still 'usable' just not able to be assigned to a host. They are used for special purposes.

Technically, all addressess are usable. We just have to define what "usable" means. Used by the network or the host or for multicasting or what?

I have a tendancy to over analyze things my wife tells me. heh.

But take 192.168.1.0/24.

It's not usable on the public internet, but is usable on a private network.

192.168.1.0 is not usable by a host, but is "used" as the network address.

192.168.1.255 is "used" by any machine needing to broadcast.

So technically, every address there is usable in some form.

My answer would be that "all" addresses are usable. It just depends upon how they are to be used.

 

[Chr1sUK]

I think he was referring to usable in terms of hosts.

I thought .0 and .255 could never be assigned to a host?

 

[bierhunter]

It depends upon the subnet. For example:

192.168.0.0/23

Network address = 192.168.0.0
Broadcast address = 192.168.1.255

Host range = 192.168.0.1 - 192.168.1.254

192.168.0.255 and 192.168.1.0 are usable as host addresses with a 23-bit subnet mask using network 192.168.0.0.

The answer to the question of usable hosts changes depending upon whether he's asking about classful networks with no subnetting, classful networks with subnetting, or classless networks.

You can also take a class-c of 192.168.0.0 and chop it in half as a /25. You suddenly lose two more usable host addresses. 192.168.0.127 becomes a broadcast address and 192.168.0.128 becomes a network address.


The teacher posing the question just needs to define the parameters in order to get a decent answer.