routing(7) Miscellaneous Information Manual routing(7)
NAME
routing - system support for local network packet routing
DESCRIPTION
The network facilities for HP-UX provide general packet routing support. Routing table maintenance is handled by application processes.
A routing table consists of a set of data structures used by the network facilities to select the appropriate remote host or gateway when
transmitting packets. The table contains a single entry for each route to a specific network or host, as displayed by the command with the
or options (see netstat(1)). Routes that are not valid are not displayed.
_______________________________________________________________
# netstat -r
Routing tables
Destination Gateway Flags Refs Use Interface Pmtu
hpindwr.cup.hp.com
localhost UH 1 39 lo0 4608
localhost localhost UH 0 68 lo0 4608
147.253.56.195 localhost UH 0 0 lo0 4608
147.253.144.66 localhost UH 0 0 lo0 4608
default hpinsmh.cup.hp.com
UG 1 21 lan0 1500
15.13.136 hpindwr.cup.hp.com
U 1 92 lan0 1500
147.253.56 147.253.56.195 U 0 7 lan2 1500
147.253.144.64 147.253.144.66 U 0 7 lan1 1500
_______________________________________________________________
# netstat -rn
Routing tables
Destination Gateway Flags Refs Use Interface Pmtu
15.13.136.66 127.0.0.1 UH 1 39 lo0 4608
127.0.0.1 127.0.0.1 UH 0 68 lo0 4608
147.253.56.195 127.0.0.1 UH 0 0 lo0 4608
147.253.144.66 127.0.0.1 UH 0 0 lo0 4608
default 15.13.136.11 UG 2 30 lan0 1500
15.13.136.0 15.13.136.66 U 1 113 lan0 1500
147.253.56.0 147.253.56.195 U 0 7 lan2 1500
147.253.144.64 147.253.144.66 U 0 7 lan1 1500
_______________________________________________________________
# netstat -rv
Routing tables
Dest/Netmask Gateway Flags Refs Use Interface Pmtu
hpindwr.cup.hp.com/0xffffffff
localhost UH 1 39 lo0 4608
localhost/0xffffffff
localhost UH 0 68 lo0 4608
147.253.56.195/0xffffffff
localhost UH 0 0 lo0 4608
147.253.144.66/0xffffffff
localhost UH 0 0 lo0 4608
default/0x00000000
hpinsmh.cup.hp.com
UG 2 31 lan0 1500
15.13.136/0xfffff800
hpindwr.cup.hp.com
U 1 129 lan0 1500
147.253.56/0xfffffe00
147.253.56.195 U 0 7 lan2 1500
147.253.144.64/0xfffffff0
147.253.144.66 U 0 7 lan1 1500
_______________________________________________________________
# netstat -rnv
Routing tables
Dest/Netmask Gateway Flags Refs Use Interface Pmtu
15.13.136.66/255.255.255.255
127.0.0.1 UH 1 39 lo0 4608
127.0.0.1/255.255.255.255
127.0.0.1 UH 0 68 lo0 4608
147.253.56.195/255.255.255.255
127.0.0.1 UH 0 0 lo0 4608
147.253.144.66/255.255.255.255
127.0.0.1 UH 0 0 lo0 4608
default/0.0.0.0 15.13.136.11 UG 3 40 lan0 1500
15.13.136.0/255.255.248.0
15.13.136.66 U 1 153 lan0 1500
147.253.56.0/255.255.254.0
147.253.56.195 U 0 8 lan2 1500
147.253.144.64/255.255.255.240
147.253.144.66 U 0 8 lan1 1500
_______________________________________________________________
The following columns are of particular interest:
The destination Internet address: host name, network name, or
The keyword indicates a wildcard route, used as a last resort if no route is specified for a particular remote
host or network. See
The netmask and the destination Internet address together define
a range of IP addresses that may be reached by the route's gateway. A host route by default has a netmask of
all 1's. A default route by default has a netmask of all 0's. The netmask is also used in selecting a route to
forward an IP packet. See the subsection.
The gateway to use to get to the destination:
a remote gateway or the local host. See
The type of route:
The route is "up" or available (see
ifconfig(1M)).
The route uses a remote host as a gateway;
otherwise, the local host is shown as the gateway (see route(1M)).
The destination is a host;
otherwise, the destination is a network (see route(1M)).
The interface connections:
The local loopback after system boot.
The interface cards installed on the local host after the
command is executed at boot time (see ifconfig(1M)).
The values of the count and destination type fields in the command determine the presence of the and flags in the display and thus the
route type, as shown in the following table.
Count Destination Type Flags Route Type
-------------------------------------------------------------
=0 network U Route to a network
directly from the local
host
>0 network UG Route to a network
through a remote host
gateway
=0 host UH Route to a remote host
directly from the local
host
>0 host UGH Route to a remote host
through a remote host
gateway
=0 default U Wildcard route directly
from the local host
>0 default UG Wildcard route through a
remote host gateway
-------------------------------------------------------------
Subnets
The network facilities support variable-length subnetting. An Internet address is made up of a portion, and a portion of an address in the
form:
Subnet addresses are defined as a portion of the network's Internet address. This scheme provides for:
o Network addresses that identify physically distinct networks.
o Subnet addresses that identify physically distinct subnetworks of the same network.
A network manager can subdivide the Internet address of the local network into subnets using the host number space. This facility allows
several physical networks to share a single Internet address.
To allow for this, three Internet classes are defined, each accommodating a different amount of network and host addresses. The address
classes are defined by the most significant bit of the binary form of the address.
The following table lists the number of networks, nodes, and the address ranges for each address class:
Nodes per
Class Networks Network Address Range
--------------------------------------------------------------
A 127 16777215 0.0.0.1 - 127.225.225.254
B 16383 65535 128.0.0.1 - 191.255.255.254
C 2097151 255 192.0.0.1 - 223.244.244.243
Reserved -- -- 224.0.0.0 - 255.255.255.255
--------------------------------------------------------------
The first 8 bits of a Class A network has network space for only 127, while accommodating the largest number of nodes possible among the
classes defined. A single class B network has the network address limitation of 16 bits, and 16 bits to define the nodes.
For example, a Class C address space is as follows:
______________________________________
Indicates Class C
Class C subnet
networks portion
| |
--- ---
10000000.00000110.00000001.11100001
-------------------------- -----
| |
Network Address Host
= 192.6.1 Address
= 1
______________________________________
A subnet for a given host is specified with the command (see ifconfig(1M)), using the parameter with a 32-bit subnet mask.
The default masks for the three classes of Internet addresses are as follows:
Class A: 255.0.0.0
Class B: 255.255.0.0
Class C: 255.255.255.0
An example Class C network number is 192.34.17.0. The last field specifies the host number. Thus, all hosts with the prefix 192.34.17 are
recognized as being on the same logical and physical network.
If subnets are not in use, the default mask used is 255.255.255.0.
If subnets are used and the 8-bit host field is partitioned into 3 bits of subnet and 5 bits of host as in the above example, then the sub-
net mask would be 255.255.255.192.
If a host has multiple interfaces, then it can belong to different subnets. Unlike past releases, the subnets can have different sizes
even if they may have the same network address. This is accomplished by using a different netmask on each of the host interfaces. For
example, the and interface shown in the tables above are connected to two distinct subnets of the same network, 147.253. The subnet that
belongs to can have at most 14 hosts, because its netmask is 255.255.255.240.
Note: The host portion of those IP addresses in the subnet cannot be all 1's or all 0's, therefore this subnet can support only 14 hosts,
not 16.
The subnet that belongs to can have up to 510 hosts, because its netmask is 255.255.254.0.
Supernets
A supernet is a collection of smaller networks. Supernetting is a technique of using the netmask to aggregate a collection of smaller net-
works into a supernet. This technique is particularly useful for class C networks. 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,
for example, 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:
o Entries for a specific host.
o Entries for all hosts on a specific network.
o 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 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 facili-
ties select the routing entry that specifies the narrowest range of IP addresses. For example, the host route entry that has a destina-
tion/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 destina-
tion/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 Such multi-
ple routes include:
o Two or more routes to a host via different gateways.
o Two or more routes to a network via different gateways.
A superuser can change entries in the table by using the command (see route(1M), or by information received in Internet Control Message
Protocol (ICMP) redirect messages.
If there are more than one default gateways for a particular net or subnet, each will be used in turn to effect the even distribution of
datagrams to the different gateways.
WARNINGS
Reciprocal commands must be executed on the local host and the destination host, as well as all intermediate hosts, if routing is to suc-
ceed in the cases of virtual circuit connections or bidirectional datagram transfers.
AUTHOR
was developed by the University of California, Berkeley.
FILES
SEE ALSO
netstat(1), ifconfig(1M), route(1M), route(7P).
routing(7)
