The routing of IP datagrams is very similar to sorting letters at a post office. There is a sorting table with slots in it. Each slot has a mail pouch attached beneath. All slots are marked with the names of the respective towns with direct mail service from the town where the post office is situated.
Sorting is done by a post office clerk, who sorts letters one by one and checks the address of every letter. If the addressee is, say, in Vienna, the clerk drops the letter into the slot marked Vienna. If the addressee is in a village near Prague, the clerk drops the letter into the slot marked Prague (since there is no direct mail service to the village near Prague and the nearest point with direct mail service is Prague). When all the letters have been dealt with, the mail pouches are detached from the sorting table. A tag with the name of the town where the pouch is to be sent is attached to each of them and the mail pouches are sent.
Figure 7.3: Manual sorting of letters in a...