The default port number for the Global Catalog in Active Directory is 3268 for standard (non-SSL) LDAP queries.

For completeness:

  • Global Catalog over LDAP (non-SSL) : port 3268.
  • Global Catalog over LDAP over SSL (LDAPS) : port 3269.

These ports are used on domain controllers that are configured as Global Catalog servers to allow forest-wide directory searches.