[last change: May 10th]
How URL number systems work
This is the note Hackmore Readrite sent me that explains URL number ranges.
He says he gleaned it from other sources. I think it's a very useful summary.
TCP/IP assumes that there are a large number of independent networks connected together by gateways. The user should be able to access computers or other resources on any of these networks. All you need to know in order to access another system is an address like 220.127.116.11. It's actually a 32-bit number, but it's written as 4 decimal numbers, each representing 8 bits of the address. The structure of the address gives the information on how to get to a system. (ex. 128.6 is the number assigned to Rutgers University.) The next octet indicates which Ethernets are involved. 128.6.4 is used by the Computer Science department. The last octet allows up to 254 systems on each Ethernet. (254 because 0 and 255 are not allowed) As mentioned, Internet addresses are 32-bit numbers, normally written as 4 octets (in decimal), e.g. 18.104.22.168. There are actually 3 different types of address. The problem is that the address has to indicate both the network and the host within the network. They thought that 24 bits would be needed to represent all the IP networks, and that some very big networks might need 24 bits to represent all of their hosts. This would seem to lead to 48 bit addresses. But the designers really wanted to use 32 bit addresses. So they adopted a kludge, assuming that most networks will be small. So they set up three different ranges of address. Addresses beginning with 1 to 126 use only the first octet for the network number. The other three octets are available for the host number. So 24 bits are available for hosts. These numbers are used for large networks. But there can only be 126 of these big networks, like Arpanet and large commercial networks are few. But not many normal organizations get a "class A" address. For normal large organizations, "class B" addresses are used. Class B addresses use the first two octets for the network number. Thus network numbers are 128.1 through 191.254. (We avoid 0 and 255, and addresses beginning with 127, because they are used for special purposes). The last two octets are available for host addesses, giving 16 bits of host address. Allowing for 64,516 computers, enough for most organizations. (You can get more than one class B address, if you run out.) Class C addresses use three octets, in the range 192.1.1 to 223.254.254. These allow only 254 hosts on each network, but there can be lots of these networks. Addresses above 223 are reserved for future use (class D and E, not currently defined). 0 and 255 have special meanings. 0 is reserved for machines that don't know their address. It's possible for a machine not to know the number of the network it is on, or even its own host address. For example, 0.0.0.23 would be a machine that knew it was host number 23, but didn't know on what network. 255 is used for "broadcast", (a message that you want every system on the network to see.) They're used in situations where you don't know who to talk to. Example, you need to look up a host name and get its Internet address, but you don't know the address of the nearest name server. So, you might send the request as a broadcast. Or if a number of systems are interested in the same information, it's less expensive to send a single broadcast than to send datagrams individually to each host that's interested. To send a broadcast, you use an address that is made using your network address, with all ones in the part of the address where the host number goes. So if you are on network 128.6.4, you would use 22.214.171.124 for broadcasts. It's not possible to send broadcasts on Arpanet, or on point to point lines. But it is possible on an Ethernet. If you use an Ethernet address with all its bits "on" (all ones), every machine on the Ethernet is supposed to look at that datagram. For convenience, the standard also allows 255.255.255.255 to be used. This refers to all hosts on the local network. Certain older implementations may use 0 instead of 255 to form the broadcast address. Such implementations would use 126.96.36.199 instead of 188.8.131.52 as the broadcast address on network 128.6.4. Certain older implementations may not understand subnets. So they consider the network number to be 128.6. They will assume a broadcast address of 184.108.40.206 or 220.127.116.11. Because 0 and 255 are used for unknown and broadcast addresses, normal hosts should never be given addresses containing 0 or 255. Addresses should never begin with 0, 127, or any number above 223.
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