SunScreen SKIP User's Guide, Release 1.5.1

Name space identifiers (NSIDs) identify the type of keys being used. SunScreen EFS 2.0 supports the following NSIDs:

NSID 0, which specifies that the IP address of the host is the key identifier for the host's X.509 certificate. Using NSID 0 results in a small improvement in encryption/decryption efficiency, since SKIP does not need to include a key identifier in each packet.
NSID 1, which is the IPv4 address assigned to the X.509 certificate by a certification authority. This IP address does not correspond to the IP address used by your computer. Rather, it is an eight-byte hexadecimal number assigned to the certificate by the certification authority to bind a unique Distinguished Name to the certificate contents. For example, a SunCA certificate might use 0a000101 (which translates to 10.0.1.1 in IP address notation) as a key identifier
NSID 8, which is the MD5 hash of the certificate's Diffie-Hellman public key.