Silicon Serial ID
The SSID, or Silicon Serial ID, is a unique identifier, usually 16 bytes long, used to identify individual WebTV/MSN TV clients. On dedicated hardware, this is assigned directly during manufacture time.
Format
On dedicated hardware, the SSID as shown to the end-user (accessible through the 411 power-off code) and sent in WTVP messages is usually formatted as a series of 16 hexadecimal digits. These SSIDs have a specific format behind them, which has yet to be figured out.
On WebTV for Dreamcast, the SSID usually starts with the bytes "1SEGA", and random alphanumeric characters following that. The random characters are usually 11 bytes long to make the resulting SSID 16 bytes long. How this is generated and stored is unknown as of writing.
Low-Level
The SSID at the low level on dedicated WebTV/MSN TV hardware is stored on a 1-Wire chip. On the Sony INT-W100 and INT-W200 units, the chip used is a DS2401 IC manufactured by Dallas Semiconductor (which is now a part of Maxim Integrated). The SSID is checksummed and if the checksum is invalid, the box will refuse to boot.