routing(7) routing(7)
NAME [Toc] [Back]
routing - system support for local network packet routing
DESCRIPTION [Toc] [Back]
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 netstat
command with the -r or -rn 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
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_______________________________________________________________
# 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:
Destination The destination Internet address: host name,
network name, or default. The default
keyword indicates a wildcard route, used as a
last resort if no route is specified for a
particular remote host or network. See
Flags.
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Netmask 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 Routing
Algorithm subsection.
Gateway The gateway to use to get to the destination:
a remote gateway or the local host. See
Flags.
Flags The type of route:
U The route is "up" or available (see
ifconfig(1M)).
G The route uses a remote host as a
gateway; otherwise, the local host
is shown as the gateway (see
route(1M)).
H The destination is a host;
otherwise, the destination is a
network (see route(1M)).
Interface The interface connections:
lo0 The local loopback
after system boot.
lan0, lan1,... The interface cards
installed on the
local host after the
ifconfig command is
executed at boot
time (see
ifconfig(1M)).
The values of the count and destination type fields in the route
command determine the presence of the G and H flags in the netstat -r
display and thus the route type, as shown in the following table.
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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 [Toc] [Back]
The network facilities support variable-length subnetting. An
Internet address is made up of a network address portion, and a host
address portion of an address in the form:
192.34.17.0
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
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:
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Nodes per [Toc] [Back]
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 ifconfig command (see
ifconfig(1M)), using the netmask 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
subnet mask would be 255.255.255.192.
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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 lan1 and lan2 interface shown in the netstat tables above
are connected to two distinct subnets of the same network, 147.253.
The subnet that lan1 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 lan2 belongs to can have up to 510 hosts, because its
netmask is 255.255.254.0.
Supernets [Toc] [Back]
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 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 [Toc] [Back]
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
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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 information received in Internet Control Message
Protocol (ICMP) redirect messages.
WARNINGS [Toc] [Back]
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 [Toc] [Back]
routing was developed by the University of California, Berkeley.
FILES [Toc] [Back]
/etc/hosts
/etc/networks
SEE ALSO [Toc] [Back]
netstat(1), ifconfig(1M), route(1M).
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