Native SQL clients on Windows will attempt to connect to the first address (IPv4 or IPv6) resolved for the host.
Note: In V11, IPv6 is supported on Windows and Linux. Java and ADO.NET SQL clients currently support only IPv4. Explicit IPv6 addresses in client connection strings are not currently supported.
Server keyword SQL_OPTION NO_IPV6 can be added to ctsrvr.cfg to accept only IPv4 connections. This is not generally recommended and is more likely to cause connection issues than to solve them.
Note: The COMM_PROTOCOL F_TCPIPV6 keyword is commented out in the default ctsrvr.cfg file. Be sure to remove the comment symbol (the semicolon - ;) before trying to use this support.
An environment variable can be used for native clients to request only IPv4 address: CTSQL_IPV4_ONLY. Setting this variable to any value in the environment will effectively disable IPv6 connection attempts from the client. This may be needed on networks where both IPv4 and IPv6 are enabled, but the c-tree SQL server does not accept IPv6 connections.