AS Numbers

AS numbers (ASNs) were originally assigned as 16-bit numbers in the range 1-64,511. The last 1000 AS numbers are reserved for private AS space, for labs, etc. It became obvious in the early 2000s that AS numbers would be exhausted within 10 years. For this reason, from 2009 onwards, 32-bit ASNs have been assigned. With long AS numbers, a dotted notation is used, either 8 or 16-bit blocks.

16bits.16bits

8bits. 8bits. 8bits. 8bits

When dealing with an ISP you must arrange getting your AS published. This is much more complex than getting a normal Internet connection, you need to begin discussions with your ISP very early on. Your ISP will provide a peer connection to their AS from your AS. You provide a static route to get to their router and their AS, there is no automatic detection of peers in BGP, BGP does not advertise. Peers must be configured on both sides. Your side will then advertise your prefixes (subnets) to the ISPs peer.

RIPE can be found at www.ripe.net and a search box exists on the home page. They also detect my public IP address (most modern networks use NAT and my final public IP address will have no relation to the network address my computer is using). If I type in my IP address RIPE tell me my ISP, the address prefix block my address is part of (78.155.224.0/19) and the AS number (AS43599).

Irish education has a single ISP, HEANet [3] with an AS number of AS1213. If we put that AS number in the search box we can see which peers HEANet have and who the transit providers are. This is static information, we might not want to trust it!

The Internet is just a very large number of inter-connected AS.

You can find a dynamic report of the current status of the Internet routing table and AS at http://www.cidr-report.org/as2.0/

To get an overview of BGP routing, we use looking glass servers; look at http://www.bgp4.as/looking-glasses. You may be able to access some of the servers listed either through a web interface or Telnet, to review the BGP routing tables. The routing table currently has over 500,000 prefixes.

When we connect to other AS, we may use default routes, but if we have multiple paths to the Internet, this will probably result in non-optimal routing. Alternatively, we could receive a partial routing table, for example if we connect to a neutral peering point. If we are buying transit from a carrier, we would expect a full routing table. The router resources to cope with routing tables this size are considerable.