IB Computer Science Topic A2.3.1: Different Types of IP Addressing Notes

IP Addresses: The Numbers That Get a Packet to the Right Device

1. Every device on a network needs an IP address so that packets can be delivered to it and any reply can find its way back.
2. Each packet carries a source IP address and a destination IP address in its header, and routers read the destination to decide where to send it next.
3. One address is described in three separate ways at once, so learn the three pairs as independent choices: IPv4 or IPv6, public or private, and static or dynamic.

IPv4 and IPv6: A 32-Bit Address Space Ran Out

1. IPv4
   1. Uses a 32-bit address, so there are 2^32 possible values, which is about 4.3 billion addresses.
   2. Written in dotted-decimal form as four numbers from 0 to 255 separated by dots, for example 192.168.1.1.
   3. The pool has effectively run out, because far more than 4.3 billion devices now connect to the internet.
2. IPv6
   1. Uses a 128-bit address, giving about 340 undecillion (3.4 x 10^38) possible addresses.
   2. Written as eight groups of four hexadecimal digits separated by colons, for example 2001:0db8:85a3:0000:0000:8a2e:0370:7334.
   3. Leading zeros in a group can be dropped and one run of all-zero groups can be replaced by a double colon, so that address shortens to 2001:db8:85a3::8a2e:370:7334.
   4. The header has 8 fixed fields instead of IPv4's 14, and IPsec support is built in rather than optional.

Public and Private Addresses: The Same Numbers Reused Behind Every Router

1. Public IP addresses
   1. Are globally unique and can be reached directly from anywhere on the internet.
   2. Are allocated by the Internet Assigned Numbers Authority (IANA) through regional registries, which pass blocks to internet service providers.
   3. Are needed by anything that must accept incoming connections from outside, such as a web server or a mail server.
2. Private IP addresses
   1. Are used only inside a LAN and are never forwarded across the public internet by routers.
   2. Come from three ranges reserved by RFC 1918: 10.0.0.0 to 10.255.255.255, 172.16.0.0 to 172.31.255.255, and 192.168.0.0 to 192.168.255.255.
   3. Can be reused by every private network in the world at the same time, which is what slows down the drain on the public IPv4 pool.
   4. Are used by the computers, printers and phones sitting behind a home or office router.

Static and Dynamic Addresses: Set Once or Leased on Demand

1. Static IP addresses
   1. Are entered manually by a network administrator and stay the same until someone changes them.
   2. Suit devices that others must find at a known address, such as web servers, mail servers, network printers and routers.
   3. Cost administrative time, because every change is made by hand and the administrator has to avoid assigning the same address twice.
2. Dynamic IP addresses
   1. Are handed out automatically by a DHCP server from a pool of available addresses.
   2. Are granted as a lease for a fixed period, so a device may be given a different address the next time it connects.
   3. Use the address pool efficiently, because an address returns to the pool when a lease expires and can then be given to another device.
   4. Suit laptops, phones and guest devices that join and leave the network constantly.

NAT: One Public Address Fronting a Whole Private Network

1. Network address translation (NAT) lets many devices holding private addresses share a single public address whenever they communicate with the internet.
2. It does two jobs at once: it minimizes the number of public IP addresses an organisation has to buy, and it lets a private internal network communicate with the public internet even though private addresses cannot be routed there.

How a NAT Router Rewrites an Address

1. A device on the private network sends a packet carrying its own private source address, for example 192.168.1.15.
2. The NAT device, normally the router, replaces that private source address with its own public address before forwarding the packet out to the internet.
3. It records the swap in a translation table, together with a port number it assigns to that particular connection.
4. The reply comes back addressed to the router's public address and the assigned port.
5. The router looks up the port in the translation table and rewrites the destination back to the correct private address before delivering it inside the network.
6. Port numbers are what keep the connections apart, so many devices can use one public address at the same moment without their traffic getting mixed up.

The Limitations of NAT

1. Unsolicited incoming connections fail by default, because a packet arriving from outside has no matching entry in the translation table.
2. Port forwarding must be set up by hand if a device inside the network needs to accept incoming connections, such as a games server or a camera.
3. Every packet needs rewriting, which adds processing work at the router and can slow a busy connection.
4. Protocols that put an IP address inside the data rather than the header break under NAT, because only the header gets rewritten.
5. Hiding the internal addresses from outside also has security consequences, which are covered under firewalls and network security.