Help regarding cisco!!?

Question

Can you give me a secret about IP addressing and Subnetting? I'm gonna take a certification Exam and that is where I REALLLLYYY need help! I can troubleshoot and config, but when it comes to IP addressing and subnetting.. I SUCK!!!! And do you know where I can get the BOSON Network simulator? thanks!!!!

Answer 1

There seem to be a lot of questions about IP subnetting, but it's really not difficult if you just break it down. So regardless of whether your really talking about classless or classfull addressing, the same mechanisms are used. So lets say we're using classfull addressing, A/B/C address space -

Class A 1.0.0.1 to 126.255.255.254
Class B 128.1.0.1 to 191.255.255.254
Class C 192.0.1.1 to 223.255.254.254

The class "A" address space uses a "network" mask of 255.0.0.0. So if we see an address of 12.10.10.10 we know that the first "octect" is the "network" portion of the address, but we need to know what "subnet mask" is being used so that we can tell what the "host" portion of the address is versus the "network.subnet" portion is. So lets say that we have 12.10.10.10/24 (call this host A). The "/24" indicates that the network+subnet portion of the address is 24 bits, in other words the upper 3 octects (mask will also be specified in the same format as an IP address - a "/24" is equal to a 255.255.255.0 mask). So lets contrast this with 12.10.12.20/24 (call this host B). Same "mask" (24 bits), and lets see if these two hosts are on the same subnet.

Remember, the network.subnet portion of the IP address is used for routing, so if the 12.10.10.10/24 hosts wants to speak to the 12.10.12.20/24 host it first needs to decide if it can just try sending the packet directly to the host, locally, or if it needs to send it to a router so that the router can figure out how to get it to the host.

So hosts A first figures out what it's own subnet (network+subnet) address is, and to that it needs to break the address into binary (it's only a computer after all). And remember, the dotted decimal notation that we use to talk about IP addresses contains 4 octects - 4 chunks of 8 bits each. So 12.10.10.10 in binary =

0000 1100 . 0000 1010 . 0000 1010 . 0000 1010 (I put dots between the octects just to help seperate them)

Now we need to apply the mask - the "/24" which means the upper 24 bits. So in binary this looks like =

1111 1111 . 1111 1111 . 1111 1111 . 0000 0000 (again, I'm throwing the dots in to help keep it clear)

Now lets see what the address looks like with the mask "applied"

1111 1111 . 1111 1111 . 1111 1111 . 0000 0000 - mask
0000 1100 . 0000 1010 . 0000 1010 . 0000 1010 - address

So only the bits of the address that are "masked", that have the mask bit set to 1 matter. So the last 8 bits of the address, because the mask bits are 0, don't matter - they are NOT part of the subnet address (we say subnet but it means the network+subnet). So now we've isolated the subnet address of the sending host, host A, now we need to compare it to the address that we want to send to, host B.

12.10.12.20 in binary =

0000 1100 . 0000 1010 . 0000 1100 . 0001 0100

Now lets apply the mask (again, upper 24 bits) -

1111 1111 . 1111 1111 . 1111 1111 . 0000 0000 - mask
0000 1100 . 0000 1010 . 0000 1100 . 0001 0100 - address

And now we can again extract the "subnet" portion of the address for host B - the upper 3 octects, and compare them to the "subnet" of Host A -

0000 1100 . 0000 1010 . 0000 1100 - host B (network+subnet)
0000 1100 . 0000 1010 . 0000 1010 - host A (network+subnet)

Do ALL of the bits match? No. This means that the source and destination addresses reside on different "subnets", and there needs to be a router involved - the host cannot just send locally.

So understand:

- Dotted decimal notation is used to represent 4 octets (from octal meaning 8) of binary data.
- Mask are expressed as either the number of bits used, starting with the upper octect, such as /16 or /24 or in the equivilant dotted decimal notation. So a "/16" in dotted decimal notation would be 255.255.0.0.
- Convert the dotted decimal address into binary.
- Convert the "mask" into binary.
- Apply the mask to the address.

So break it down to binary, thats what computers do. A computer will do as above and then perform a logical AND function on the 2 binary addresses to determine if they match or are different.

Answer 2

The best resource for learning IP subnetting is the Network Plus certification training CD by "Transcender" or the Cisco Press Routing and Switching for CCIE book.

I personally used the transcender CD's.

Answer 3

you can get neywork simulator from www.mcmcse.com
and a very good documentation on subnetting and ip addressing

Answer 4

Check out www.firewall.cx its got all that you need to know about subnetting and CISCO.

Answer 5

We r on the same boat Dude!

Answer 6

thought you meant the thong song guy!

Answer 7

I have no idea

Answer 8

don't know much about that stuff so sorry

Answer 9

these may help