IPv4 address structure and special ranges

What an IPv4 address is

An IP address (Internet Protocol address) is the logical address of a device on a network. It is used by routers to send packets from one network to another. IPv4, version 4 of the protocol, uses 32-bit addresses written as four octets in dotted decimal, like 192.168.1.20. With 32 bits there are 2 to the power 32 combinations, about 4.3 billion addresses in total.

Every address has two parts. The network part says which network the device belongs to. The host part says which device it is on that network. A separate number, the subnet mask, tells you where the network part ends and the host part begins. You will learn masks in the next topic.

The old classes: A, B, C, D and E

In the early internet, the first octet decided the size of the network. This is called classful addressing. You no longer configure networks this way, but the CCNA exam still expects you to know the classes, and many people still say "a class C network" when they mean a /24.

Classful addressing wasted huge numbers of addresses. A company that needed 300 addresses was too big for a class C (254 hosts) and got a class B with 65,534 hosts. In 1993 the internet moved to CIDR (Classless Inter-Domain Routing): any prefix length can be used, and the mask is always given with the address. That is the system used today.

Private addresses (RFC 1918)

RFC 1918 sets aside three ranges for private use inside homes, offices and campuses. Anyone can use them, and they are never routed on the public internet. Your home Wi-Fi router almost certainly uses one of them.

Why private addresses and NAT exist

4.3 billion addresses sounded huge in 1981, but today there are far more phones, laptops and servers than that. IANA, the body at the top of address allocation, handed out its last free IPv4 blocks in 2011, and the regional registries, including APNIC for India and the Asia-Pacific, ran short soon after. Two fixes keep IPv4 going: private addresses, which thousands of organisations reuse internally, and NAT (Network Address Translation), where a router swaps many private addresses for one or a few public addresses on the way out. The long-term fix is IPv6, covered in the next module.

Other special addresses

• Loopback, 127.0.0.0/8: traffic to these addresses never leaves the device. 127.0.0.1 means "this computer". ping 127.0.0.1 tests your own network software.
• APIPA (Automatic Private IP Addressing), 169.254.0.0/16: a Windows or Mac PC gives itself one of these when it cannot reach a DHCP server. Seeing 169.254.x.x on a PC almost always means "DHCP failed".
• 0.0.0.0: means "this host, no address yet" when a PC asks DHCP for an address. In routing, 0.0.0.0/0 means "every network", the default route.
• 255.255.255.255: the limited broadcast. A packet sent here goes to every device on the local network and is never forwarded by a router.
• Documentation ranges 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24: reserved for books and examples, so this course uses them for "public" examples.

Unicast, broadcast and multicast

Unicast is one sender to one receiver, the normal case. Broadcast is one sender to every device on the local network, for example a DHCP Discover. Multicast is one sender to a group of receivers that asked to join, using addresses from 224.0.0.0/4 (224.0.0.0 to 239.255.255.255). Routing protocols use multicast: OSPF routers talk to 224.0.0.5, for example.

import ipaddress

for text in ["192.168.1.20", "127.0.0.1", "169.254.10.5", "224.0.0.5", "172.32.0.1"]:
    ip = ipaddress.ip_address(text)
    print(text, ip.is_loopback, ip.is_link_local, ip.is_multicast, ip in ipaddress.ip_network("172.16.0.0/12"))

Common mistakes

• Giving a server on the internet a private address and wondering why nobody outside can reach it: private addresses need NAT to reach the internet.
• Ignoring a 169.254.x.x address. It is a symptom, not a configuration: fix the DHCP problem (cable, VLAN, DHCP server, relay).
• Saying 127.0.0.1 is the only loopback address. The whole 127.0.0.0/8 block is loopback.