Networks in More Detail
Addresses and names on a network
Read a machine's address, work out which machines share a network, and say what turns a name into an address.
Machines find each other by number and people find things by name, and both are true at once, which is why there is a whole service whose only job is turning one into the other. The number is called an address, and it is written as four smaller numbers with dots between them, because that is easier for a person to copy down than the long run of bits it really is. A message can only be sent to an address. If all you have is a name, nothing can be sent yet — the name has to be traded in for a number first, and that trade is a request across the network like any other.
The front of an address says where, the back says which
An address is not one thing but two things stuck together. The front of it names a network, and every machine on that network shares those numbers exactly. Whatever the network has not already used up is the part that tells one machine on that network apart from another, so no two machines sharing a network may have the same one. This is the single fact worth carrying out of this skill, because it is what a device is really asking when it looks at an address: is the front of this the same as the front of mine? If it is, the message stays put and is delivered locally. If it is not, the message has to leave.
Worked example
The first three numbers name the network. Is 172.18.6.4 on the same network as 172.18.9.4?
Compare the first numbers: 172 and 172. The same so far.
Working left to right is what makes this quick — the moment two differ you are finished, and you never have to look at the rest.
Try it together
Now work through a network of your own. Every address on it begins 10.24.7, and the last number may be anything from 30 to 45.
Answer yes or no where the question asks for it.
1.How many machines can this network hold?
Have a go
Have a go on your own. Each of the four numbers runs from 0 to 255. Which of these could never be a machine's address?
Ready to practice?
Eight questions on what you have just read. Nothing is timed, and you can play as many times as you like.