ifconfig.1m (2011 09)

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ifconfig(1M) ifconfig(1M)
A primary interface is an interface whose IP index number is zero. A secondary interface is an interface
whose IP index number is non-zero.
Loopback Interface
The loopback interface (
lo0) is automatically configured when the system boots with the TCP/IP
software. The IP address and netmask of the primary IPv4 loopback interface are 127.0.0.1 and
255.0.0.0, respectively. The IP address and prefix of the primary IPv6 loopback interface are ::1 and 128
respectively. The user is not permitted to change the address of the primary loopback interface (
lo0:0).
It is permissible to assign other IP addresses to lo0 with non-zero IP index numbers (lo0:1, lo0:2, etc).
This allows a system to have a "system IP" address that is available as long as one interface remains
usable.
Supernets
(inet only) A supernet is a collection of smaller networks. Supernetting is a technique of using the net-
mask to aggregate a collection of smaller networks into a supernet.
This technique is particularly useful when the limit of 254 hosts per class C network is too restrictive. In
those situations a netmask containing only a portion of the network part may be applied to the hosts in
these networks to form a supernet. This supernet netmask should be applied to those interfaces that con-
nect to the supernet using the
ifconfig command. For example, a host can configure its interface to
connect to a class C supernet, 192.6, by configuring an IP address of 192.6.1.1 and a netmask of
255.255.0.0 to its interface.
IPv6 Interfaces
inet6 must be specified when an IPv6 interface is configured. The address for an IPv6 interface can either
be a hostname present in the host name database (see hosts (4)), or an address in the IPv6 colon notation.
Stateless Address Auto-configuration
Unlike IPv4 interfaces, IPv6 interfaces can be configured without an address and/or a prefix. Stateless
address autoconfiguration requires no manual configuration of hosts, minimal (if any) configuration of
routers, and no additional servers.
A primary interface (
lanX:0) is automatically assigned a link-local address by the system when the
interface is configured. A link-local address comprises the well-known link-local prefix FE80::/10 and the
interface identifier, which is typically 64 bits long and is based on EUI-64 identifiers. The link-local
address allows automatic discovery of other hosts and routers on the same link, using the Neighbor
Discovery Protocol (see NDP(7P)). The link-local address can be used as the source address to communi-
cate with other nodes when no routers are present.
If a router on the local link advertises prefixes in router advertisements, the host autoconfigures its secon-
dary interfaces and its default gateway. The address of an autoconfigured secondary interface is formed
by prepending the prefix received from the router to the interface identifier, the same interface identifier
that is used in forming the primary interface.
Manual Address Configuration
IPv6 interfaces can also be configured with manually assigned addresses and/or prefixes. A primary
interface must be configured with a link-local address and the prefix must not be specified. The prefix is
always 10. The universal/local bit, the U bit, of the interface identifier must be 0, per section 2.5.1 of
RFC 2373. Accordingly, a manually assigned address for a primary interface must have the following
pattern:
FE80::xMxx:xxxx:xxxx:xxxx where x is any hexadecimal digit, and M must be 0, 1, 4, 5, 8,
9, C, or D.
When a primary interface is configured with a manually assigned address, secondary interfaces will be
autoconfigured if the host receives prefixes from router advertisements. The addresses on the secondary
interfaces will be derived from the interface identifier portion of manually configured address in the pri-
mary interface.
When a secondary interface is configured with a manually assigned address, and if the user chooses an IP
index number that has been used for an autoconfigured secondary interface, the manual configuration
overwrites the autoconfiguration. When this happens, network connectivity through the overwritten
autoconfigured IP address is temporarily lost. At a later time, when the host receives the next router
advertisement, the host will bring up another secondary interface with a different IP index number, but
with the same IP address, and network connectivity through that IP address is restored. Normally, a
user can avoid this by checking used IP index numbers. However, there is always a possibility that
address autoconfiguration due to router advertisement is happening concurrently while the user
4 Hewlett-Packard Company − 4 − HP-UX 11i Version 3: September 2011