Method and apparatus for controlling the routing of data packets in an IP network (200). A DNS system (202) stores a packet admission policy configured for a first end-host (B) that dictates conditions for allowing other end-hosts to get across data packets to the first end-host or not. A routing voucher is defined which is required for routing data packets to the first end-host. The routing voucher is distributed to routers (R) in the IP network. When an address query is received at the DNS system (202) from a second end-host, the voucher is supplied to the second end-host if the configured policy allows the second end-host to convey data packets. Otherwise, the voucher is not supplied. If allowed, the second end-host will add the routing voucher to any data packets directed to the first end-host. When a valid routing voucher is present in a packet at a router (204) in the network, the packet will be forwarded to the next router in the IP network. The router will otherwise discard the packet.