IPv4 addresses
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Every device on a network needs an address so messages can find it. Take one real-looking address, 192.168.1.20, apart: four numbers, each made of 8 bits, split into a network part (the street) and a host part (the house). Then meet the special ranges: private, loopback and public.
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01Why can each of the four numbers in an IPv4 address only go up to 255?
One octet is 8 bits. 2 to the power 8 is 256, so the values run from 00000000 (0) to 11111111 (255).
02What is the 8-bit number 10101000 in decimal?
Add up the place values that hold a 1: 128 + 32 + 8 = 168. That is the second number of 192.168.1.20.
03In 192.168.1.20/24, which part is the network part?
/24 means the first 24 bits (the first three numbers) name the network. The last 8 bits, 20, name the device (the host).
04Which of these addresses is private (RFC 1918)?
172.16.0.0/12 runs from 172.16.0.0 to 172.31.255.255, so 172.20.4.1 is private. 172.40.0.1 is outside that range.
05You send a message to 127.0.0.1. Where does it go?
127.0.0.0/8 is loopback: it always means this device. Programmers use it to talk to programs on their own computer.
المسرد
- IP address
- A number that names a device's connection to a network, so messages can be sent to it.
- IPv4
- Version 4 of the Internet Protocol. Its addresses are 32 bits long.
- Bit
- The smallest piece of data: a single 0 or 1.
- Octet
- A group of 8 bits (a byte). An IPv4 address is four octets.
- Dotted decimal
- The usual way to write an IPv4 address: four numbers from 0 to 255 with dots between them.
- Prefix length
- The /n after an address. It says how many leading bits are the network part, e.g. /24.
- Private address
- An address from 10.0.0.0/8, 172.16.0.0/12 or 192.168.0.0/16. Reused in many homes and offices, never routed on the internet.
- Loopback
- 127.0.0.0/8, usually 127.0.0.1 (localhost): an address that always means this device.
الملاحظات والمصادر
- IPv4 addresses are 32 bits long (RFC 791). The /n prefix notation comes from CIDR (RFC 4632).
- 192.168.1.20, 192.168.1.1 and 192.168.1.35 are private RFC 1918 addresses; 203.0.113.5 is an RFC 5737 documentation address standing in for a real public one.
- 127.0.0.0/8 is reserved for loopback by RFC 1122 §3.2.1.3; the special-purpose ranges are listed in RFC 6890.
- Prefix lengths do not have to fall between the dots (for example /20); the subnets lesson shows how that works.
- NAT (RFC 3022) and IPv6 (RFC 8200) each get their own lesson.
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