The IPv6 header contains a hop limit field that controls the number of hops over which a datagram can be sent before being discarded. This is similar to the TTL field in the IPv4 header. An application can influence the value of the hop limit field using the following options:

• IPV6_UNICAST_HOPS socket option (hop limit value to be used for unicast packets on a socket)
• IPV6_MULTICAST_HOPS socket option (hop limit value to be used for multicast packets on a socket)
• IPV6_HOPLIMIT ancillary data option on sendmsg() (hop limit value to be used for single packet)

For a unicast packet, the following precedence order is used to determine a packet's hop limit value:

1. If IPV6_HOPLIMIT ancillary data is specified on sendmsg(), use its value.
2. If the IPV6_UNICAST_HOPS socket option is set, use its value.
3. If a router advertised hop limit is known, use its value.
4. If there is a globally configured IPv6 hop limit, use its value.
5. Use the IPv6 default unicast hop limit, 255.

For a multicast packet, the following precedence order is used to determine the packet's hop limit value:

1. If IPV6_HOPLIMIT ancillary data is specified on sendmsg(), use its value.
2. If the IPV6_MULTICAST_HOPS socket option is set, use its value.
3. Use the IPv6 default multicast hop limit, 1.