With 16-bit Autonomous System Numbers (ASNs), 65,536 unique numbers are possible. Just like 32-bit IP addresses, these 16-bit ASNs are becoming a scarce resource. Therefore, in 2007 the Internet Engineering Task Force (IETF) developed a new format, 32-bit ASNs (RFC 4893), which increases the supply of ASNs to four billion.

The RIPE NCC started assigning 32-bit ASNs (or 4-byte ASNs as they are also called) to interested parties in January 2007. Since January 2010, the RIPE NCC has assigned 16 and 32-bit ASNs from a single pool. Upon request, however, the RIPE NCC still assigns 16-bit ASNs.

In Figure 1 below, you can see the distribution of 32-bit ASNs compared to 16-bit ASNs the RIPE NCC assigned since 2007 (Please note that 2011 only include numbers for January—July 2011).

Figure 1: 16-bit (blue) compared to 32-bit (red) ASN assignments by the RIPE NCC

In 2011, roughly one third of all ASNs assigned by the RIPE NCC consisted of 32- bits. This means that even with 32-bit ASNs available since 2007, quite a number of 16-bit ASNs are still being requested. However, as you can see in Figure 2, 54% of all 32-bit ASNs handed out globally have been assigned by the RIPE NCC.

Figure 2: Global 32-bit ASN Distribution by RIR

If operators are using up-to-date equipment and software and their upstream provider supports 32-bit ASNs, they should not experience any problems. The RIPE NCC itself started using 32-bit ASNs in 2007 and has not experienced any problems.

However, as Figure 3 shows, 25% of all 32-bit ASNs assigned by the RIPE NCC, were returned. The main reason for this is that either the user or the upstream provider’s equipment did not support 32-bit ASNs. This is becoming less of a problem as time goes by.

Figure 3: Distribution of RIPE NCC assigned 32-bit ASNs returned, visible and not visible in BGP

Figure 3 shows that 52% of the 32-bit ASNs assigned by the RIPE NCC are visible in the global routing table, as compared to the 23% that are not yet visible. When comparing this with 16-bit ASNs, we found that the visibility of 32-bit ASNs is lagging slightly behind. Overall, we can conclude that the majority of the RIPE NCC membership does not see any operational problems with 32-bit ASNs. Please also see Assigning 32-bit ASNs on RIPE Labs.