The heart of the routing problem is to figure out which IMP to send a packet that is not meant for you, but is simply passing through. For example, suppose that a packet originates at a host attached to IMP A and is destined for a host attached to IMP I. We will use the shortcut by saying that the packet originates at A and is destined for I. Since there is no direct connection between A and I, it must go through some intermediate IMPs, and there are numerous choices.

Every router has a table that says "If an incoming packet is destined for X, then send it to Y" where X is an IMP that is not directly connected to this router and Y is one that is.