If the IPv6 MCP run-time key is present, TCP/IP initializes in dual mode, where TCP/IP is IPv4–capable and IPv6–capable. If the IPv6 MCP run-time key is not present, TCP/IP initializes in IPv4 only operating mode and a waiting entry similar to the following warns the user that the key is not present and that the system is running in IPv4 only mode.

4233/4393 75 8:29 TCPIP/WARNING/TASK/ACCEPT/0
ACCEPT: The IPv6 key is missing or expired. IPv6 Run-Time Key is
required. Running in IPv4Only Mode. TCPIP will need to be restarted
for IPv6 operation. *** ENTER: 'AX OK', OR DS.

If TCP/IP is IPv6–capable and you want to restrict TCP/IP operation and configuration to IPv4 only, set the IPV4OnlyOperation option of the TCPIP OPTION (TCPIP OPT) command as follows:

NW TCPIP OPT + IPV4OnlyOperation

This command must be the first command processed by TCP/IP during initialization; it sets the operating mode context for processing subsequent configuration commands. This command is provided as a safeguard to prevent configuration commands that either explicitly configure IPv6 interfaces or enable IPv6 address autoconfiguration from being unintentionally issued.

In IPv4 only operating mode, commands that attempt to configure IPv6 interfaces receive the “Invalid operating mode or Command processed with exceptions” negative response, depending on the type of command and the TCPIP context at the time of processing.

If the TCPIP OPT command is not the first command in the initialization file, the command is rejected as an invalid phase.