A MES is called from the terrestrial network by its Inmarsat Number (IMN) which is identical to its public telephone number with a country code prefix according to Table 2. Internally the system uses two different (24 bits) IDs:
The Forward MESID, used on the forward path, i.e., in the direction from the Land Earth Station (LES) to the Mobile Earth Station (MES).
The Return MESID, used on the return path, i.e., from the MES to the LES.
These IDs are unrelated to the publicly published IMN and are only known to the Inmarsat system, which maintains a database describing the relationship between the IMN and the MESID pairs. These IDs are hard coded into the satellite terminals at manufacturing time.