Subnet Classes

What is the difference between subnet clases, for example: 255.255.255.255 is a Class A subnet (I think anyway) and 255.255.255.0 is a class B subnet.

What do they actually mean??
:confused:

subnet don't have classes.. IP's do.

ips from 1 to 127 are Class A Ip's. usualy the base subnebt mask is 255.0.0.0 wich allows you to have 1.7 million hosts.

from 128 to 191 are the Class B IP's the usual subnet mask is 255.255.0.0 these allow you to have parox 65 thousand hosts.

From 192 to 255 are Class C IP's, the usualy subnet mask is 255.255.255.0 these classes allow for 253 hosts.

subnet masks that are not among the above are part of a subnet, it's a little lenghty to explain, but an easy concept to understand.

Now in the above scale, there are a few "reserved" IP's.. the two common ones are 127.0.0.0 and 192.168.0.0

What are 127.0.0.0 & 192.168.0.0 reserved for?

127.0.0.1 is the local loopback. if you ping it you ping yourself. 192.168.0.0 172.16.0.0 10.0.0.0 are all private reserved address. they are not internet routable. you use them on your internal network to protect yourself from being directly accessible from the internet. I think they were also implemented in case we ran out of public IPs too soon.

When you refer to IP addresses 255.255.255.255 is a broadcast and 0.0.0.0 refers to all networks. if I don't have an explicit route for an ip it's considered 0.0.0.0 and is usually forwarded to a gateway. if I don't know the mac address of a host I send out a broadcast on IP 255.255.255.255. this is what's called an ARP. routers block arps (unless they have a specific reason not to).

a subnet mask is applied to an ip to determine the network number. 192.168.1.0/24 means 192.168.1.0 255.255.255.0 That defines a network number of 192.168.1.0 where hosts 192.168.1.1 through 192.168.1.254 are on the same network and considered local. in this scenario 192.168.2.0 would be considered a seperate network and if you wanted to reach it your packet would travel through a router.

Where it gets complicated is masking like 192.168.1.0/25 or 192.168.1.0 255.255.255.128

In this scenario you've turned your one network of 254 hosts into two networks of 126 hosts each. Your network numbers are 192.168.1.0 and 192.168.1.128 Your broadcast addresses become 192.168.1.127 and 192.168.1.255

As you add bits to the subnet you increase the number of networks and decrease the number of hosts. As you do this you've got to remember that your adding one bit. That bit is equal to 128, then 64, then 32, then 16, then 8, then 4, then 2, then 1. So, that's how you come up with subnet masks of 128, then 192, then 224, then 240, then 248, then 252, 254, 255. Except that you can't use 255 or 254 (cuz there's no hosts) and if you use 252 you only get one host.

Some of you will say no, you get two hosts with a 252. Well, technically yeah but, since you require a router your only left with one host. Most ISPs use .252 networks to implement their routers.

Now, go sit down, have a drink and keep reading it over and over. You'll get it. It's one of those things that (at least I did) you have to look at a bunch of times and then you go "oh, that's simple."

Thanx for the detailed explanation Iate.. I was too lazy to type it out :p

I can vouch for all that, just took the first Cisco Networking course and everything is correct, but like iateyourcat said, have some good quiet time to read that over and over until you get it. It isn't all that complicated, actually it's a very organized way of dealing with the ip's. Good Luck!!!

You might want to have a look here :http://www.learntosubnet.com/
very nice explanation [via windows media player]helped me get my head round the subject for night school [ much better than the tutors explanation too].
Also has a full CD for $16.95 with extras included.

Ya learntosubnet.com explains subentting very well. He also has presentations on TCP/IP and The OSI model.