IP addresses, ports and reverse DNS

Every machine on the internet has an IP address, and every service on a machine listens on a port. Together they say where a connection goes: 203.0.113.10:25565 is port 25565 at the address 203.0.113.10.

An IPv4 address is four numbers from 0 to 255, such as 203.0.113.10. An IPv6 address is longer and written with letters and digits, such as 2001:db8::10. DNS turns names into these addresses, so people rarely type them (Domains and DNS).

The addresses you order at Coritan are IPv4. DNS records and load balancers also take IPv6 addresses that you host elsewhere (IPv6). You meet three kinds of address:

An instance's primary address

The IPv4 address that comes with a Cloud Compute instance. It ends when the instance ends (Manage an instance's addresses and names).

A shared address

The address of the machine a Container Apps server runs on. Every server on that machine uses it, each on ports of its own (Manage server ports).

A floating IP

An address that belongs to your account. You attach it to an instance or a server, and can move it to another one later, so the address outlives the machine behind it. A subnet is a block of them (Floating IPs).

Traffic to a floating IP passes through DDoS Shield, and traffic to a shared address does not (DDoS attacks and protection).

A port is a number from 1 to 65535. One address can carry many services at once, each on its own port, and the port tells the machine which service a connection is for. These are ports you may meet:

When a service uses its usual port, people can leave the port out: a browser uses 443 for an https:// address, and Minecraft uses 25565. Many servers share one shared address, so each one gets port numbers of its own, and players type the port after the address. With a floating IP attached, a server can also answer on the game's usual port, so players join with the address alone (Manage server ports).

DNS turns a name into an address. Reverse DNS goes the other way: a lookup of an address returns a hostname, from a PTR record that the holder of the address sets. Mail servers check it before they accept mail, so a mail server needs reverse DNS that names a hostname, and an A record for that hostname that points back at the address.

At Coritan, you set the reverse DNS of a floating IP on its Reverse DNS tab. You can change it as often as you like, but you cannot remove it once it is set (Set reverse DNS for a floating IP).

• Order a floating IP
• Attach and detach a floating IP
• Set reverse DNS for a floating IP
• Manage server ports