HP-UX Reference (11i v1 05/09) - 1M System Administration Commands A-M (vol 3)

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ifconfig(1M) ifconfig(1M)
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, respec-
tively. 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 netmask
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::0 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 communicate with other nodes when no routers are present. If a router on the local link adver-
tises prefixes in router advertisements, the host autoconfigures its secondary 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 inter-
face 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 manually
configures secondary interfaces.
To disable communication through a specific IP address on an autoconfigured secondary interface, that
secondary interface should be marked down, not removed or overwritten with a different IP address. If
that interface is removed or overwritten, the host will reconfigure another secondary interface with the
same IP address when it receives the next router advertisement. Alternatively, the router can be
configured to stop advertising the prefix that corresponds to the offending IP address.
HP-UX 11i Version 1: September 2005 − 3 − Hewlett-Packard Company Section 1M−−349