About IP Addresses
Note: Intermapper now supports 128-bit IPv6 addresses. Most of the information in this topic is still relevant and accurate. In addition, you can enter an IPv6 address anywhere in Intermapper that you can enter a 32-bit IPv4 address.
What is an IP address? How do I get one?
An IP address ("Internet Protocol address") is a number that represents a single unique computer on the Internet. IP addresses are similar to telephone numbers, in that each computer (or telephone) must have its own unique IP address (telephone number.) Like telephones, there's a directory system - called the Domain Name System, or "DNS" - that can convert a name such as "www.apple.com" into a corresponding numeric IP address.
32-bit IPv4 Addresses are written as a sequence of four numbers separated by ".", like this: 208.123.246.35. Each of the four numbers in the IP address can take the value between 0 and 255.
Intermapper now supports 128-bit IPv6 addresses.
Every computer on the Internet must have a unique IP address. ISPs purchase large blocks of consecutive IP addresses, and then allocate smaller ranges of these addresses to their customers. Thus, a particular company might be assigned all the 254 IP addresses in the range 208.123.246.1 to 208.123.246.254. (The addresses ".0" and ".255" are not usually assigned.) Companies then assign the IP address to individual computers within the organization.
How do computers send data through the Internet?
Computers send information through the Internet by dividing the data to send into small chunks ("packets") and transmitting them to the other device. All this happens without your doing anything - the web browser, e-mail program, etc. all take care of these low level details.
When your computer wants to send to another computer, it creates the packet, then places the other computer's address in the destination address of the packet, places its own address in the source address of the packet, and then sends the packet off, either directly to the destination computer, or to a nearby router that takes responsibility for routing the packet.
There's an analogy with the post office here. Packets are like envelopes, with destination addresses and return addresses. Routers are like post offices: they check the destination address and have the responsibility for delivering the packet to the final destination computer or to another router that's closer to the destination.
What is a subnet? Why do I care?
A subnet is a range of IP addresses. The special attribute of a subnet is that all the computers within the subnet (a "sub-network") can talk directly to each other, and don't need a router to communicate.
As mentioned above, your computer delivers a packet directly to the destination computer or sends it to the router for ultimate delivery.
But how does your computer know whether the packet's destination is within its subnet? The answer is that your computer uses the subnet mask to determine the members of the subnet.
The chart below associates the number of IP addresses in a subnet to the subnet mask. For example, the subnet mask "255.255.255.0" represents 254 consecutive IP addresses. If your computer's IP and the destination computer's IP addresses are in the same subnet address range, then they can send packets directly to each other. If they're not in the same range, then they must send their data through a router for delivery.
What does the "/24" mean? How does that relate to my subnet mask?
Intermapper uses a shorthand notation to represent an IP subnet's information. The number in the "/xx" shorthand stands for the number of bits (technically, bits set to one) in the subnet mask. The convention is always to start at the left end of the 32-bit (IPv4)subnet mask. The table below shows the correspondence between the "/xx" notation and the actual numeric representation.
| Subnet Mask | # of Addresses | Subnet Mask | # of Addresses | |||
|---|---|---|---|---|---|---|
| /1 | 128.0.0.0 | 2.1 billion | /17 | 255.255.128.0 | 32,766 | |
| /2 | 192.0.0.0 | 1 billion | /18 | 255.255.192.0 | 16,382 | |
| /3 | 224.0.0.0 | 536 million | /19 | 255.255.224.0 | 8,190 | |
| /4 | 240.0.0.0 | 268 million | /20 | 255.255.240.0 | 4,094 | |
| /5 | 248.0.0.0 | 134 million | /21 | 255.255.248.0 | 2,046 | |
| /6 | 252.0.0.0 | 67 million | /22 | 255.255.252.0 | 1,022 | |
| /7 | 254.0.0.0 | 34 million | /23 | 255.255.254.0 | 510 | |
| /8 | 255.0.0.0 | 17 million (Class A) | /24 | 255.255.255.0 | 254 (Class C) | |
| /9 | 255.128.0.0 | 8.4 million | /25 | 255.255.255.128 | 126 | |
| /10 | 255.192.0.0 | 4.2 million | /26 | 255.255.255.192 | 62 | |
| /11 | 255.224.0.0 | 2.1 million | /27 | 255.255.255.224 | 30 | |
| /12 | 255.240.0.0 | 1 million | /28 | 255.255.255.240 | 14 | |
| /13 | 255.248.0.0 | 524 thousand | /29 | 255.255.255.248 | 6 | |
| /14 | 255.252.0.0 | 262 thousand | /30 | 255.255.255.252 | 2 | |
| /15 | 255.254.0.0 | 131 thousand | /31 | 255.255.255.254 | RFC 3021 | |
| /16 | 255.255.0.0 | 65,534 (Class B) | /32 | 255.255.255.255. | Loopback address |
What is a "private IP address range"?
The Internet Assigned Numbers Authority (IANA) has reserved several blocks of IP addresses that an organization may assign for its own private internet. These blocks are defined in RFC 1918 (http://www.ietf.org/rfc/r fc1918.txt?number=1918).
From the RFC:
3. Private Address SpaceThe Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private internets:
10.0.0.0 - 10.255.255.255 (10/8 prefix)
172.16.0.0 - 172.31.255.255 (172.16/12 prefix)
192.168.0.0 - 192.168.255.255 (192.168/16 prefix)We will refer to the first block as "24-bit block", the second as "20-bit block", and to the third as "16-bit" block. Note that (in pre-CIDR notation) the first block is nothing but a single class A network number, while the second block is a set of 16 contiguous class B network numbers, and third block is a set of 256 contiguous class C network numbers.