12.5. IP addresses¶

A hardware address picks a device out from the others on the same local cable or radio cell. It is fine for that job, but it is useless beyond the local segment: a switch in the next building cannot route by it, because the underlying mechanism that lets a switch learn MAC addresses only works inside one segment.

The network layer solves that with a second kind of address that is independent of which cable a device is plugged into. The name for this kind of address is the Internet Protocol address, or IP address, and the “Internet Protocol” half names the set of rules every host on the internet follows when it sends or forwards one. The current internet uses two versions of the addressing scheme in parallel – IPv4 (the older form, still dominant on small networks) and IPv6 (the newer form, slowly replacing it).

12.5.1. What an IP address is¶

An IP address is a number large enough to identify any device on the internet uniquely. It is written in a human-readable form that the rest of the section uses, but in the packet header it is just a fixed-size integer.

IPv4 addresses are 32 bits long and are written as four decimal numbers separated by dots, each number being one byte:
```
192.168.1.42
8.8.8.8
10.0.0.1
```
Thirty-two bits gives roughly four billion possible addresses, which sounded like plenty when IPv4 was designed in the 1970s and was not enough by the early 2010s.
IPv6 addresses are 128 bits long and are written as eight groups of four hexadecimal digits separated by colons:
```
2001:0db8:85a3:0000:0000:8a2e:0370:7334
```
Runs of zeros can be shortened to ::, and leading zeros in a group can be dropped, so the address above is normally written 2001:db8:85a3::8a2e:370:7334.

The two address families are otherwise different languages; an IPv4 host cannot directly send a packet to an IPv6 host without help from a gateway. The camera’s network and socket modules support both. This tutorial uses IPv4 in examples because most local networks the camera will join are still IPv4-only, but everything that follows works exactly the same way for IPv6 once the addresses are swapped out.

12.5.2. What an IP address is for¶

The IP address says which host on the internet a packet is meant for. A router that does not know how to reach a destination directly knows that some other router probably does, and forwards the packet there. The packet hops between routers, each one a little closer to the destination, until a router that is on the destination’s local segment delivers the final hop.

That hop-by-hop behaviour is what makes the internet work as one big network instead of many small islands. The next page covers how the hops are chosen; this one is just about the address.

12.5.3. How a camera gets one¶

A camera that has just joined a Wi-Fi network needs an IP address before it can talk to anything. There are two common ways for that to happen.

The first is automatic assignment. The camera asks the local network for an address; the device that hands one out is the router – the box that connects the local network to the wider internet. In most home and small-office setups, the same physical box also acts as the switch wired devices plug into and as the Wi-Fi access point wireless ones associate with, so “the router”, “the switch”, and “the access point” may all be the same piece of hardware. The router runs a small service called DHCP (the Dynamic Host Configuration Protocol), which keeps a pool of available addresses, picks one for each newly-arrived device, and leases it for a fixed time. While DHCP is at it, the router also hands the camera a couple of other useful pieces of configuration:

the address to send outbound traffic to when the destination is off the local network (the default gateway, which is the router’s own address); and
the addresses of one or more name servers that turn human-readable names like example.com into IP addresses. The name-lookup service is called DNS, the Domain Name System, and Names and DNS covers it in detail.

All of this happens automatically while the link is coming up. The camera does not have to ask for any of it explicitly; the moment isconnected() returns True in the example on the previous page, the cam already has its address, its gateway, and its name servers.

The second option is static configuration. Some deployments want a known address for the camera so that other devices can reach it without first looking it up. The ipconfig() method sets the address, the gateway, and the name server by hand:

wlan.ipconfig(addr4=("192.168.1.50/24", "192.168.1.1"))
wlan.ipconfig(dns="192.168.1.1")

Static configuration is fragile (two devices accidentally given the same address conflict). Reach for the DHCP default unless a specific reason to override it shows up.

Once the camera has an IP address, it has joined the internet (or at least the local network’s piece of it). Other devices can now address packets to it by that address, and it can address packets to them.

12.5.4. The netmask and the `/24`¶

The /24 at the end of the address in the static example above is the netmask. An IP address by itself does not say where the local network ends – 192.168.1.50 could be one of a few hundred addresses on a small home network, or one of thousands on a larger one. The netmask says how much of the address names the network and how much names the host inside it.

/24 means “the first 24 of the 32 bits name the network; the last 8 name the host”. For 192.168.1.50/24 that splits the address into 192.168.1 for the network and .50 for the host, leaving room for about 254 devices on the same local network. /16 would leave more bits for the host half and fit far more devices on one network; /30 would leave only two host addresses and fit a point-to-point link.

The netmask is also commonly written as a four-byte number in the same dotted notation as the address. /24 is equivalent to 255.255.255.0 – read each byte as “all the bits that belong to the network half”. The two forms are interchangeable; the ipconfig() reader in the next subsection happens to return the four-byte form.

Why the split matters at all – how a device uses the netmask to decide whether a destination is on the local network or needs to go out through the gateway, and why most home networks land on /24 – is covered on Private networks and NAT.

12.5.5. Reading the address back¶

The ipconfig() method without arguments returns the active configuration. The addr4 view returns the IP address and netmask:

>>> wlan.ipconfig("addr4")
('192.168.1.50', '255.255.255.0')