routing(7)routing(7)NAMErouting - system support for local network packet routingDESCRIPTION
The network facilities for HP-UX provide general packet routing sup‐
port. Routing table maintenance is handled by application processes.
A routing table consists of a set of data structures used by the net‐
work 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 sub‐
section.
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 gate‐
way;
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:
· Network addresses that identify physically distinct networks.
· Subnet addresses that identify physically distinct subnetworks of
the same network.
A network manager can subdivide the Internet address of the local net‐
work 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 accommodat‐
ing 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 lim‐
itation 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 ifcon‐
fig(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 spec‐
ifies 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 subnet
mask would be 255.255.255.192.
If a host has multiple interfaces, then it can belong to different sub‐
nets. 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 dis‐
tinct 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 net‐
mask 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
networks 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 net‐
mask 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:
· 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 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 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 multiple
routes include:
· Two or more routes to a host via different gateways.
· 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 Pro‐
tocol (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 destina‐
tion host, as well as all intermediate hosts, if routing is to succeed
in the cases of virtual circuit connections or bidirectional datagram
transfers.
AUTHOR
was developed by the University of California, Berkeley.
FILESSEE ALSOnetstat(1), ifconfig(1M), route(1M), route(7P).
routing(7)
