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 routing(7)							  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 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
      _______________________________________________________________

      # 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



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 routing(7)							  routing(7)




		      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.

	   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



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 routing(7)							  routing(7)




			       default has a netmask of all 0's.  The
			       netmask is also used in selecting a route to
			       forward an IP packet. (see Routing
			       Algorithm.)

	   Gateway	       The gateway to use to get to the destination:
			       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-
							up.

				    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.

	   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




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 routing(7)							  routing(7)




	    >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 network address portion, and a host
      address portion of an address of 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:

				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:









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 routing(7)							  routing(7)




		      _____________________________________
		      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.

      If a host has multiple interfaces, then it can belong to different
      subnets.	Unlike the 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 cannot support 16 hosts, but only 14 hosts
      instead. The subnet that lan2 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
      networks into a supernet. This technique is particularly useful for



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 routing(7)							  routing(7)




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




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 routing(7)							  routing(7)




 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

 SEE ALSO
      netstat(1), ifconfig(1M), route(1M).







































 Hewlett-Packard Company	    - 7 -   HP-UX Release 11i: November 2000