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Internet routes are specified for an address prefix. The shorter the prefix, the more general the route. A shorter prefix covers more address space and thus a bigger part of the Internet. Very long prefixes cover few addresses and are used for local routing close to the destination address. In general, it is not necessary to distribute very long prefixes to the whole Internet, because a more general, shorter prefix is sufficient to direct packets in the direction of the destination.

In order to limit the growth of the Internet routing table, many operators limit the length of the prefixes they propagate. Prefix length filtering is a common mechanism in BGP to reduce the number of entries in the global routing table. We were interested to see which prefix lengths network operators currently filter out.

The graph below shows all IPv4 prefixes seen on the routing table by the RIPE NCC Routing Information Service (RIS) peers on 1 August 2011. Each column represents one prefix length.

You can see that the majority of prefixes announced on the Internet are /24s. About half of them are visible by at least 80% of our peers (as indicated by the light green bar).

When we looked closely at the data, we found that visibility gradually decreased as the prefixes get longer, i.e. more specific.

The next graph shows IPv6 prefixes. Here, we were especially interested to see if /48s were filtered more often than /32s.

You can see that /32s and /48s make up the huge majority of prefixes, and a significantly higher percentage of prefixes are seen by all of our peers. After looking at the data in more detail, we observed that the trend seen in IPv4 of gradually decreasing visibility is not present in IPv6. Visibility drops dramatically for prefixes longer than a /48.

We found that most /32s are visible by 100% of our peers, but a large number of /48s are only visible by 93% of our peers. This means that /48s are filtered more often as /32s.

You can find more graphs and a description of the methodology in the background article on RIPE Labs: Visibility of Prefix Lengths in IPv4 and IPv6.

By Daniel Karrenberg, Chief Scientist at the RIPE NCC

Related topics: IP Addressing, IPv6