Question : Tcp/ip

Hi all
I've been trying to teach myself the principles of  Tcp/Ip including subnet calculations.  After reviewing it several time I'm catching on,  But something just isn't right here   This is probably a simple question for most of you, but it's got me scratching my head and here it is.      If i go to "Whatismyip.com"  it returns back my ip = 64.19.nnn.nnnn and it does corresponds to the ip address issued to us by our isp.  

Now since the first Octet is 64 is this considered a class A network? I'm sure were not paying for one and it's more likely our Isp is the one with the Class A address.   So can anyone clear up my confusion or where am i getting lost?  Thank you in advance for your input

Rob

Answer : Tcp/ip

I don't think cmihelich understands the question.

>Now since the first Octet is 64 is this considered a class A network

mike is pretty close but he's missing a little history (and I have no idea why he thinks there's an "E" in CIDR).  IIRC, the network "Classes" were a legacy construct implemented so that you didn't have tiresome subnet masks associated with every IP address (This was before IP space got valuable, and such a wasteful system was eventually refined).  Classes allowed folks to know what the subnet mask was simply by IP address, and was only really useful at the core (devices at the core could make routing decisions based on class, rather than having to know the subnet mask for every network).  Folks with a Class A would still chop up the network into subnets within their own organizations.

When mike says
>What that means is that your ISP uses an address range that starts with 64, thats it

he's not quite right - an IP that starts with 64 indicates a Class A, but a Class A does not indicate an IP that starts with 64 (he fell prey to logical transposition - we can forgive him I think).  He *is* correct in saying that you don't have to worry about it (unless it's going to be on a quiz or something).

Also, let me clarify a bit about CIDR - CIDR just means that when we mention a network address, we're also going to mention the subnet mask, always (even at the core).  It means that no one is going to be able to assume a specific subnet mask just by looking at the IP address, with the benefit of being able to specify more exact IP ranges at every routing level in order to improve efficiency of IP address space assignments and usage.  The only other thing you need to know about CIDR is it (AFAIK) introduces the concept of representing a subnet mask using slash ("/") notation representing the number of bits in the network portion of the address, rather than the traditional 4 octets (255.0.0.0 = /8, 255.255.0.0 = /16, 255.255.255.0 = /24, 255.255.255.240 = /28, etc).

So, my IP assignment from my ISP is 12.250.43.12/32 (not really, but I'm not posting my IP here).  This means I have one usable IP (even though 12.x.x.x is Class A).  My IP assignment at the office is 68.236.142.42/29 (not really, but I'm not posting our subnet here).  This means I have a network of 8 IPs (you do know about the network and broadcast addresses being reserved, right?  This means I really only have 6 IPs to play with, and I'll have to burn one for my router, leaving me with only 5 to use for connected machines.)

Does this help?

Cheers,
-Jon

P.S.  Want a handy way to figure out how many IPs a particular subnet mask represents?  Just subtract the final octet from 256 - ie 255.255.255.224 ->  256 - 224 = 32 IPs.  (You can adjust this logic for masks wider than 255.255.255.0 if you know what you're doing).