A lot of the AMPRNet address space is interconnected via gateways. These are IPENCAP (IP Protocol Number 4) encapsulated tunnels that carry the AMPRNet address space allocated to a particular region or end user. There exists a database of all the gateways public IP addresses and the subnets they service on the portal. This database is used to dynamically generate gateway information via modified RIP advertisements. This database also generates a file called encap.txt which is basically a routing table that specifies which subnets can be reached via which gateway.
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In order to keep this database up to date, everyone that operates a gateway must register on the portal and have their gateway assigned to their account.