One big room, four small ones
A college puts everything in one big space. The finance staff, the teachers, the students and the guests all share it.
Everyone hears everyone else's noise. A guest's infected laptop can reach the finance computers.
The technician splits it into four smaller groups. This is subnetting.
Key words
| Word | What it means |
|---|---|
| Subnetting | splitting one network into smaller networks |
| Subnet | one of the smaller networks |
| Subnet mask | a number that shows which part of an address is the network |
| Prefix length | how many bits of the address are the network part, such as /26 |
| Host bits | the bits left over for naming devices |
| Borrowing bits | taking bits from the host part to make more subnets |
| Network address | the first address in a subnet, which names the subnet itself |
| Broadcast address | the last address in a subnet, used to talk to every device in it |
| Usable address | an address a device can really use |
| Network | computers and devices joined together so they can share data |
| Router | a device that passes packets between networks |
| Packet | a small piece of data sent over a network |
| Firewall | a device or program that blocks unwanted traffic |
| Binary | a number system with two digits, 0 and 1 |
The cells students fill in are left blank in this preview.
By the end of this lesson
Note
I can split a /24 network into equal subnets, and say how many devices each can hold, using whole numbers and tables.
Starter
Have a go
Starter
3 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.
Why subnet?
Smaller networks are better. Traffic that is not for you stays out of your subnet, so the network is quieter. A virus in one subnet is held back from the others, because a router or firewall sits between them.
You can also give each group its own rules: finance, teachers, students and guests.
The idea: borrow bits
A /24 network has 24 network bits and 8 host bits. To split it, you borrow bits from the host part and give them to the network part.
Borrow 1 bit and you get 2 subnets. Borrow 2 bits and you get 4. The number of subnets doubles with each bit.
Each borrowed bit halves the size of each subnet.
Splitting a /24: the whole table
| Prefix | Bits borrowed | Subnets | Addresses in each | Usable hosts | Mask |
|---|---|---|---|---|---|
| /24 | 0 | 1 | 256 | 254 | 255.255.255.0 |
| /25 | 1 | 2 | 128 | 126 | 255.255.255.128 |
| /26 | 2 | 4 | 64 | 62 | 255.255.255.192 |
| /27 | 3 | 8 | 32 | 30 | 255.255.255.224 |
| /28 | 4 | 16 | 16 | 14 | 255.255.255.240 |
| /29 | 5 | 32 | 8 | 6 | 255.255.255.248 |
| /30 | 6 | 64 | 4 | 2 | 255.255.255.252 |
The cells students fill in are left blank in this preview.
Check
Have a go
Check
4 questionsYour answers are marked here and are not kept. A college account keeps every student’s marks.