HP-UX Reference (11i v1 00/12) - 5 Miscellaneous Topics, 7 Device (Special) Files, 9 General Information, Index (vol 9)

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STANDARD Printed by: Nora Chuang [nchuang] STANDARD

/build/1111/BRICK/man7/!!!intro.7

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routing(7) routing(7)

netmask is 255.255.254.0.

Supernets

A supernet is a collection of smaller networks. Supernetting is a technique of using the netmask to aggre-

gate a collection of smaller networks into a supernet. This technique is particularly useful for class C net-

works. A Class C network can only have 254 hosts. This can be too restrictive for some companies. For

these companies, a netmask that only contains a portion of the network part can be applied to the hosts in

these class C networks to form a supernet. This supernet netmask should be applied to those interfaces

that connect to the supernet using the ifconﬁg command (see ifconﬁg(1M)). 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.

Routing Algorithm

The routing table entries are of three types:

• Entries for a speciﬁc host.

• Entries for all hosts on a speciﬁc network.

• Wildcard entries for any destination not matched by entries of the ﬁrst two types.

To select a route for forwarding an IP packet, the network facilities select the complete set of "matching"

routing table entries from the routing table. A routing table entry is considered a match, if the result of the

bit-wise AND operation between the netmask in the routing entry and the IP packet’s destination address

equals to the destination address in the routing entry.

The network facilities then select from the set the routing entries that have the longest netmask. The

length of a netmask is deﬁned as the number of contiguous 1 bits starting from the leftmost bit position in

the 32-bit netmask ﬁeld. In other words, the network facilities select the routing entry that speciﬁes the

narrowest range of IP addresses. For example, the host route entry that has a destination/netmask pair of

(147.253.56.1, 0xffffffff), is more speciﬁc than the network route entry that has a destination/netmask pair

of (147.253.56.0, 0xfffffe00), therefore the network facilities select the host route entry. The default route by

default has a destination/netmask pair of (0,0). Therefore the default route matches all destinations but it is

also the least speciﬁc. The default route will be selected only if there is not a more speciﬁc route.

There may still be multiple routing entries remaining. In that case the IP packet is routed over the ﬁrst

entry displayed by

netstat -r. Such multiple routes include:

• Two or more routes to a host via different gateways.

• Two or more routes to a network via different gateways.

• Two default routes.

A superuser can change entries in the table by using the route command (see route(1M), or by informa-

tion received in Internet Control Message Protocol (ICMP) redirect messages.

WARNINGS

Reciprocal

route commands must be executed on the local host and the destination host, as well as all

intermediate hosts, if routing is to succeed in the cases of virtual circuit connections or bidirectional

datagram transfers.

AUTHOR

routing was developed by the University of California, Berkeley.

FILES

/etc/hosts

/etc/networks