IP address is the device's unique identifier in the network. It works on 3rd layer of OSI, The Network layer. It uses Best-effort delivery, meaning it has no guarantee data won't be lost on the way, which is mitigated by The Transport layer protocols.

It has two common standards: IPv4 and IPv6.

IPv4 addresses can be represented in any notation by 32-bit number.

IP addresses can be public and private. Private IP addresses:

• 10.0.0.0/8
• 172.16.0.0/12
• 192.168.0.0/16

There's other reserved ranges:

• 0.0.0.0/8 - current network
• 100.64.0.0/10 - shared address space used by ISPs
• 127.0.0.0/8 - loopback interface
• 169.254.0.0/16 - link-local addresses (for when IP is not specified, like when retrieving it from a DHCP server)
• 192.88.99.0/24 - formerly used for IPv6 to IPv4 relay
• 255.255.255.255/32 - limited broadcast
• …and so on.

IPv6 was intended to replace IPv4. It uses 128-bit addresses. Direct communication between IPv6 and IPv4 is impossible, though there are some transition mechanism to help with that.

IPv6 simplifies many aspects, so it's faster than IPv4.