Autonomous System Numbers or ASNs are provided by Internet registries to Internet Service Providers as a unique identifier of their network. ISPs may have multiple networks and therefore multiple ASNs, one for each non-contiguous subnet, which may get confusing for network managers or customers, as you can see in this example below. ASNs are also used to configure Border Gateway Protocol (BGP) between two networks or between two subnets.
MxToolbox has developed an ASN Lookup Tool that takes an ISPs name and returns the ASNs they own. In addition, the tool can accept an ASN and return the name of the owner. This tool can be useful for network managers working with large ISPs or trying to determine ownership of a particular ASN or attempting to setup BGP.