Addressing Modes
IP addresses can be used in several different ways.
- Unicast and anycast addresses come from the same shared pool. The difference is how they are deployed in production.
- Every IP subnet has both a broadcast address.
- Multicast is used behind the scenes on most networks for services like local DNS. This uses the .local Top-Level Domain, which is reserved for local use. It lets you access a machine with a name like “alices-tablet.local” instead of typing an IP address like 192.168.53.178.
| Unicast | One-to-one communication. This is the most common form of address usage on the internet. Examples of unicast deployments include video conferencing, online computer games, and browsing websites. In each case, one end of the communication sends data in packets. Each packet goes to a single recipient. |
| Anycast | Similar to unicast, but the service is distributed in many locations. Communication is handled by the nearest service. DNS is a widely used example of anycast. For instance, the root DNS server operated by the RIPE NCC has just one IPv4 address but 125 sites. Some of those sites have multiple servers answering DNS queries. The internet’s routing protocol, BGP, sends queries to the nearest site. This gives users a resilient and rapid service as individual sites can go offline for maintenance without taking the whole service down. |
| Broadcast | A broadcast address reaches all IPv4 addresses on a network or network segment. IPv6 does not implement broadcast. Broadcast is used in DHCP, the protocol for assigning IPv4 addresses to devices when they connect to a network. When it connects, the device sends a broadcast, which is received by every device on that network segment. The DHCP server will respond to the broadcast. |
| Multicast | These are special IPv4 addresses that represent groups of IP addresses. Every listening address gets a copy of the data packet sent to the multicast address. Multicast is used behind the scenes to make local naming work. It is also used to distribute video in some cable TV networks, and pricing information on financial exchanges. Every network with an AS Number has a block of 256 multicast addresses. These are called GLOP addresses. GLOP is not an acronym, the creators just liked the sound of the term. Every network with a /24 of IPv4 unicast addresses also has a multicast address. Anyone without an AS Number or a /24, or who needs more, can request multicast addresses from IANA, the internet’s central registry. There is no charge for multicast address assignments. |