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Networking

Package: IP_PACKAGE. Registered by createEngine(); for a slimmer engine, register it explicitly (see choosing packages).

Every device on a network has an IP address, four numbers from 0 to 255 written with dots, like 192.168.1.10. Addresses are handed out in blocks called subnets, written as an address followed by a slash and a number, like 192.168.1.0/24. That number, the prefix, says how many of the address’s bits are fixed to name the block; the rest are free to number the machines inside it. A /24 fixes the first 24 bits (the first three numbers), leaving the last for hosts, so 192.168.1.0/24 is the block from 192.168.1.0 to 192.168.1.255.

These are the questions a network note keeps asking, answered in place rather than worked out elsewhere and pasted back:

hosts in 192.168.1.0/24 // 254
netmask of /24 // 255.255.255.0
broadcast of 192.168.1.0/24 // 192.168.1.255
192.168.1.10 in 10.0.0.0/8 // false
10.0.0.0/8 as int // 167,772,160
  • hosts in a block is how many machines fit in it. It is two fewer than the block’s size, because the first address names the network itself and the last is the broadcast address, so neither is given to a machine.
  • netmask of a prefix is the same split shown as an address: the fixed bits as 1s. /24 fixes three numbers, so its netmask is 255.255.255.0. You can ask about a bare prefix (/24) or a whole block.
  • broadcast of a block is its last address, the one that reaches every host at once.
  • <address> in <block> asks whether an address belongs to a block: it is true when the address shares the block’s fixed bits. 192.168.1.10 is not in 10.0.0.0/8, because that block only holds addresses starting with 10.
  • as int is the address as the single 32-bit number it really is, handy when a tool wants the integer form.

The boundary

Section titled “The boundary”

This covers IPv4, the dotted-quad addresses above. IPv6, the longer addresses written with colons, is left for a later addition: its 128-bit addresses need their own notation and their own arithmetic, and the dotted-quad form is the common case.

A dotted address only reads as one when it is written as a single run, with no spaces around the slash. That keeps ordinary division working: 192.168.1.0/24 is a subnet, but 192.168.1.0 / 24, with spaces, is a division, and a plain 10 / 2 is always just 5. A part above 255 is not a valid address either, so it is never mistaken for one.