I need help bad!!!!!!

[computerchick]

Hi everyone I'm new here and I really need some friends to help me understand networking a little better than what my professor is teaching me. Right now I'm learning about Subnets. Please can someone explain to me how you borrow bits to create two subnets. I love computers but i don't understand them in depth and would really love to.

[pbyhistorian]

There are a couple of short articles at TechRepublic.com that may help. Links tend to change over time, so I'll give you the title:
Cisco IP subnetting 101: Five things you should know
The section titled
What do 1s and 0s have to do with it
tells you adding 1s to the end of the subnet mask supplied to you creates new subnets.

The article
Cisco IP subnetting 101: Five more things you should know
gives you a simple formula (2 to the nth power, where n is the number of 1s added to the end of the supplied subnet mask) that tells you one more 1 will create two subnets.

Finally, the article
TechRepublic Tutorial: Subnetting a TCP/IP network
shows how your subnet mask is combined with an(y) address to figure out which subnet the address belongs to, near the end of the section titled
Learning how subnetting works
The router would then know to which interface (network cable) to send packets with that address.

So if your company was assigned a typical Class C network, the subnet mask would be 255.255.255.0 (11111111.11111111.11111111.00000000). You could split that range of 256 addresses into two subnets (one for each of your router's two Ethernet interfaces, for example) of 128 addresses by adding one more 1 to the Class C subnet mask (11111111.11111111.11111111.10000000 or 255.255.255.128). Assigning to a router's interface an address from a particular subnet tells the router that that interface "owns" that subnet. For example, assigning aaa.bbb.ccc.1 to interface Ethernet1 using a subnet mask of 255.255.255.128 tells the router any packets addressed to aaa.bbb.ccc.0 to aaa.bbb.ccc.127 should be sent out interface Ethernet1. Assigning aaa.bbb.ccc.129 to interface Ethernet2 with the same subnet mask tells the router that packets addressed to the other half of the Class C (aaa.bbb.ccc.128 to aaa.bbb.ccc.255) should be sent out interface Ethernet2. Of course, if the packet originates in the same subnet to which it's addressed, the router does nothing since the recipient has already seen the packet (through the hub or switch connecting the computers to the router).

That's how it worked ages ago. If I've made a mistake, someone will correct me.

(ex-Marietta, GA)

[computerchick]

:wth k so i have to break down an ip address to binary form first and after that i break down the 255.255.255.128 the same way but after that what and where do you borrow from?

[pbyhistorian]

Well, I've never heard "borrow from" when talking about subnetting. I assumed that you meant "borrow bits from the host addresses".

For example: a Class C has 24 bits of mask (11111111.11111111.11111111.00000000 or 255.255.255.0), leaving 8 bits (the "0" or "don't care" bits) for host addresses. With 8 bits, you can have 256 host addresses (0-255) in your Class C network. If you "borrow" 1 bit from the portion used for host addressing, you get 25 bits of mask - 11111111.11111111.11111111.10000000 (255.255.255.128) - and only 7 bits for host addresses. This transforms one Class C network into two (sub)networks capable of 128 hosts each.
(For those who don't know, 256 = 2^8 and 127 = 2^7. 2 is used because that's how many values a bit can have. Eight bits can create 256 combinations of 1s and 0s. Also: don't worry about host addresses that start with one or more 1s confusing the router; you drew the line between the network portion and the host-address portion when you specified the net mask.)

I can't think of any other meaning for "borrow from" - only borrowing the leftmost bit(s) from the host-address portion of the net mask supplied by your ISP.