Can you please explain to me how the octets in an IP are broken down?

[MichaelD]

I'm trying to figure something out...can you help?

I remember reading about how an IP address, example "221.34.234.97" is broken down. Something about the first two octets are the top level or something like that. Then the third and fourth stand for something else. Can you explain to me how it works?

Also, if I see an IP like this: "221.34.234.97.122" does that mean he's coming in on port 122, or that's the port he's going out on?

 

[notfred]

123.123.123.123 <- IP address

123.123.123.123:80 <- port 80 at 123.123.123.123

 

[SSP]

The octets serve a purpose other than simply separating the numbers. They are used to create classes of IP addresses that can be assigned to a particular business, government or other entity based on size and need. The octets are split into two sections: Net and Host. The Net section always contains the first octet. It is used to identify the network that a computer belongs to. Host (sometimes referred to as Node) identifies the actual computer on the network. The Host section always contains the last octet. There are five IP classes plus certain special addresses:

Meh

 

[MichaelD]

Ahh, cool! Nice linky, SSP. Thanks guys.

 

[Siva]

there can also be a 3rd section, for subnets, which is bascially to keep a LAN organized.

the network part of an address in a class A address is xxx.000.000.000 with 0s representing the host (an example of someone/thing that would own a class A address would be an ISP).
the network part of a class B address is xxx.xxx.000.000 (this would be for the ISP's node)
and for the class C address, it is xxx.xxx.xxx.000, this would be a smaller node.


i don't know about D and E

 

[MaxDSP]

I had a wee bit o' trouble with subnetting in class last term. After many repeated attempts to learn it, I am now at like 70% with this subnetting stuff.

 

[Siva]

jesus, i'm like 2% with subnetting 😛

 

[Viper GTS]

Subnetting really isn't that bad, I'm not fast at it but I understand it & can figure it out.

The speed comes from a subnet calculator.

😀

Viper GTS

 

[Kadarin]

CIDR rocks... but for the most part I just use a handy subnet chart. In a typical /24 network, the first three octets identify the network (i.e. the 192.168.1.0/24 network), and the last octet identifies the node (i.e. 192.168.1.100). It only really gets confusing when you're talking about /29 and /30 (or some other number) networks..

woot.

 

[HamSupLo]

I have a quetion too:

suppose the network portion of an address is: 192.168.12.x. A company would get a block of addresses that starts with 192.168.12.x and then they can assign IPs to themselves like from 192.168.12.1 - 192.168.12.40? and the subnet will tell them how many nodes they can get?

 

[Viper GTS]

Originally posted by: jooksing
I have a quetion too:

suppose the network portion of an address is: 192.168.12.x. A company would get a block of addresses that starts with 192.168.12.x and then they can assign IPs to themselves like from 192.168.12.1 - 192.168.12.40? and the subnet will tell them how many nodes they can get?

Yes.

192.168.x.y is a private subnet, though, so a company can have as many IPs as they want. But they could use subnetting to divide it up.

For example, a subnet of 255.255.255.248 allows for 8 IPs per block.

Why?

The mask is:

11111111 11111111 11111111 11111000

2^3 = 8 possible IPs for that block.

A 255.255.255.240 subnet would allow for 16 IPs.

Of course a network & broadcast address would be reserved, but the block size is 2 raised to the number of zeros.

Viper GTS

 

[MaxDSP]

Originally posted by: Viper GTS
Subnetting really isn't that bad, I'm not fast at it but I understand it & can figure it out.

The speed comes from a subnet calculator.

😀

Viper GTS

Subnet Calculator? I need me one of those.

 

[ScottMac]

IP addresses are defined into classes.

A class "A" IP address starts with 00xxxxxx (binary) in the first octet and goes to 01111111 (127) in the first octet.
A class "B" IP address starts with 10xxxxxx (binary) in the first octet and goes to 10111111 (191) in the first octet.
A class "C" IP address starts with 11xxxxxx (binary) in the first octet and goes to 11011111 (223) in the first octet
A class "D" IP address starts with 111xxxxx (binary) in the first octet and goes to 11101111 (240) in the first octet
A class "E" IP address starts with 1111xxxx (binary) in the first octet and goes to 11111111 (255) in the first octet

A "natural" mask for class A is 8 bits, 255.0.0.0 or /8
A "natural" mask for class B is 16 bits, 255.255.0.0 or /16
A "natural" mask for class C is 24 bits, 255.255.255.0 or /24

Class D is assigned for multicast, Class E is "experimantal" (and broadcast).

Regardless of what mask is applied, a class A address is always a class A address, because of the starting bits of the first octet; likewise for class B, C, D, E.

When a mask is extended beyond the "natural" mask into the host portion of the address, the extention becomes the "subnet adddress" or "subnet bits."

Conventionally, a network can be subnetted out to 30 bits (255.255.255.252) which allows for (by convention) two hosts. This is most frequently used in point-to-point WANS, where there only two hosts (the endpoint routers) on the segment.

FWIW

Scott