Anycasting allows you to have multiple servers in different data centers all share the same IP. IRCNow currently uses BuyVM, which provides anycasting free of charge for customers that purchase 3 VMs with them provided they are all in different data centers.
The benefit is that, whenever a user connects to your network, he will request from the server that is closest to him, reducing the latency of the connection.
To set up anycasting, first follow the instructions provided by your provider (here are BuyVM's), then configure OpenBSD to use those IP addresses.
Suppose you have the normal IP address 18.104.22.168 and 10.0.1.1 as the anycast IP address. Inside /etc/hostname.if0, you should put:
Suppose you have
inet 22.214.171.124 255.255.255.0 inet alias 10.0.1.1 255.255.255.255
Make sure to configure this for each of the three servers that share the same anycast IP address.