Subnetting

Argh.

Doing subnets is easy... explaining subnetting is hard....

Perhaps if you could be a bit more specific about what you're having trouble with?

How does it increase the number of available hosts?

Shortage of IPv4 addresses is something that keeps coming up.

I'm doing my degree by correspondence. All of our communication with other students and our instructor is through a subject forum. We've got a professor this semester who doesn't even read the forum and simply refuses to take any notice of questions. Last semester our prof was there every day answering questions and guiding students. I'm absolutely disgusted with this guy.

I know that if you've got an IP address of 192.158.10.8 and a mask of 255.255.255.0 then

11000000.10011110.00001010.00001000
11111111.11111111.11111111.00000000

The 1's in the mask denote the network part of the address (this is a class C address) and the 0's point to the host part. But when you start stealing bits from the host part and it starts talking about an extended network address, my brain starts to fade.

OK.



Looking at the subnet mask, you can see that the block of addresses to which your particular host belongs is 192.158.10.0 through 192.158.10.255. You've got 256 addresses, two of which are not usable, so you can have 254 devices. (The base network address, 192.158.10.0, and the broadcast address, 192.158.10.255, can't be assigned to devices.)

If you have several locations, you can break up the block of addresses into smaller subnets. You have more networks, but fewer addresses in each network.

If you needed four smaller networks, you could change your subnet mask to be 255.255.255.192. This divides your subnet into four blocks of 64 addresses (two unusable in each block).

The last octet of the subnet mask changes from 00000000 to 11000000. And the two highest-order bits in the IP address are now used to determine which subnet the address is in:

00xxxxxx (addresses 0-63)
01xxxxxx (addresses 64-127)
10xxxxxx (addresses 127-191)
11xxxxxx (addresses 192-255)

So those two bits that used to be part of the host portion of the address are now used for the network address. it extends the number of bits that are used to determine the network portion of the address, thus extended network address.

And a block of addresses that would have gone mostly unused is now utilized across four locations, thus conserving IPv4 addresses.

Clear as mud, I know.

Let me know what I need to clarify.

Best description I've read so far. cheers

0 to 63 would be 1 to 63 yes?

No. 64 addresses. (0, 1, 2, 3, ... 60, 61, 62, 63)

Zero and 63 are not useable though, so 62 usable, 1-62.

This is the last thing I need to get.

The usable addresses....1-62
64-126
128-191
193-254

?????
I know I'm slow, but since I want to major in network engineering, I really need to get this!

Once you subnet, 0, 64, 128, and 192 will all be network base addresses and all unusable. 63, 127, 191, and 255 will all be network broadcast addresses and all unusable.






Imagine a network 172.16.0.0 with a subnet of 255.255.0.0

Then, 172.16.0.0 is the base, 172.16.255.255 is the broadcast.

172.16.128.0 would be a valid address to assign to a host. The computers don't care that the humans think it's odd. To the computer, it's just a series of 32 bits.

2 a.m. here, I'm off to bed.

I've got it. Thanks for your effort and input.

I got 4/4 for the subnetting question and a "great answer" comment. Your input was really helpful.

Glad I could help!