About
If you want to use TCP/IP to send a data packet, the network layer (in the OSI layer model or the IP layer) must determine if and how the target IP address can be accessed.
Routing Table
The routing information necessary for this is stored in the routing table.
Route
The entire route to the target address is not described in the routing table, instead, only the route up to the next node (next hop) to which the data packet is forwarded.
There, a check is made in one of its own routing tables as to how the target address should be accessed.
Direct
This is called an indirect route, since the route goes through at least one gateway.
Indirect
If the address can be directly accessed, this is called a direct route.
Example
Simple Route
In this example, the three hosts are connected together by network A and network B:
A host is usually addressed by its host name. If Host 3 makes a connection to Host 1, the target address 192.5.5.7 (R1) is addressed. By default, this connection goes over network A. If you want the connection to the same target address to run over network B instead, you must have a corresponding routing entry in Host 3. You may want to route the connection over network B, for example, to increase performance (network B is faster than network A). For communication to run in both directions over network B, there must also be a corresponding routing entry for Host 3 on Host 1.
Management
tracing
route tracing, see:
Tools
UNIX
The above example described an indirect host route. The target address is a complete IP address (a host route), and it is also accessed through a gateway using the IP name R1B (an indirect route). On UNIX hosts, you can display the routing table by using the command netstat –r. For indirect routes, the flag G is used, and for host routes, the flag H is used. To create an indirect route, you must specify in the command route add a Metric (hop count) greater than 0.
| Action | Command |
|---|---|
| Create route on host 1: | route add R3 R3B 1 |
| Create route on host 3: | route add R1 R1B 1 |
| Display all static routes: | netstat –r |
| Test routes: | traceroute |
Host 3:
| Destination | Gateway | Flag |
|---|---|---|
| R1 | R1B | UGH |
Host 1:
| Destination | Gateway | Flag |
|---|---|---|
| R3 | R3B | UGH |
Netstat Routes
netstat -r
Kernel IP routing table
Destination Gateway Genmask Flags MSS Window irtt Iface
default 172.19.0.1 0.0.0.0 UG 0 0 0 eth0
172.19.0.0 0.0.0.0 255.255.0.0 U 0 0 0 eth0
Windows NT
On Windows NT hosts, you can display the routing table by using the command route print. Indirect routes are indicated by a Metric = 1 and are made defaults with the command route add.
| Action | Command |
|---|---|
| Create route on host 1: | route add R3 R3B 1 |
| Create route on host 3: | route add R1 R1B 1 |
| Display all static routes: | route print |
| Test routes: | tracert |
Host 3:
| Network address | Gateway address | Metric |
|---|---|---|
| R1 | R1B | 1 |
Host 1:
| Network address | Gateway address | Metric |
|---|---|---|
| R3 | R3B | 1 |
Route PRINT
route PRINT
===========================================================================
Interface List
0x1 ........................... MS TCP Loopback interface
0x2 ...00 50 56 c0 00 08 ...... VMware Virtual Ethernet Adapter for VMnet8
0x3 ...00 50 56 c0 00 01 ...... VMware Virtual Ethernet Adapter for VMnet1
0x4 ...02 00 4c 4f 4f 50 ...... Microsoft Loopback Adapter
0x5 ...00 1c 23 33 c8 42 ...... Broadcom NetXtreme 57xx Gigabit Controller - Pac
ket Scheduler Miniport
0x6 ...00 1d e0 8d 1b 21 ...... Intel(R) Wireless WiFi Link 4965AGN - Packet Sch
eduler Miniport
0x10008 ...00 ff 02 a9 5b 2d ...... TAP-Win32 Adapter V8 - Packet Scheduler Mini
port
===========================================================================
===========================================================================
Active Routes:
Network Destination Netmask Gateway Interface Metric
0.0.0.0 0.0.0.0 192.168.1.254 192.168.1.145 20
10.0.0.0 255.255.255.0 10.0.0.6 10.0.0.6 30
10.0.0.6 255.255.255.255 127.0.0.1 127.0.0.1 30
10.10.10.0 255.255.255.0 10.10.10.10 10.10.10.10 30
10.10.10.10 255.255.255.255 127.0.0.1 127.0.0.1 30
10.255.255.255 255.255.255.255 10.0.0.6 10.0.0.6 30
10.255.255.255 255.255.255.255 10.10.10.10 10.10.10.10 30
127.0.0.0 255.0.0.0 127.0.0.1 127.0.0.1 1
192.168.1.0 255.255.255.0 192.168.1.1 192.168.1.1 20
192.168.1.0 255.255.255.0 192.168.1.145 192.168.1.145 20
192.168.1.0 255.255.255.0 10.0.0.1 10.0.0.6 1
192.168.1.1 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.1.145 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.1.255 255.255.255.255 192.168.1.1 192.168.1.1 20
192.168.1.255 255.255.255.255 192.168.1.145 192.168.1.145 20
192.168.12.0 255.255.255.0 192.168.12.1 192.168.12.1 20
192.168.12.1 255.255.255.255 127.0.0.1 127.0.0.1 20
192.168.12.255 255.255.255.255 192.168.12.1 192.168.12.1 20
224.0.0.0 240.0.0.0 10.0.0.6 10.0.0.6 30
224.0.0.0 240.0.0.0 10.10.10.10 10.10.10.10 30
224.0.0.0 240.0.0.0 192.168.1.1 192.168.1.1 20
224.0.0.0 240.0.0.0 192.168.1.145 192.168.1.145 20
224.0.0.0 240.0.0.0 192.168.12.1 192.168.12.1 20
255.255.255.255 255.255.255.255 10.0.0.6 6 1
255.255.255.255 255.255.255.255 10.0.0.6 10.0.0.6 1
255.255.255.255 255.255.255.255 10.10.10.10 10.10.10.10 1
255.255.255.255 255.255.255.255 192.168.1.1 192.168.1.1 1
255.255.255.255 255.255.255.255 192.168.1.145 192.168.1.145 1
255.255.255.255 255.255.255.255 192.168.12.1 192.168.12.1 1
Default Gateway: 192.168.1.254