Previous Page | Next Page

  1. Introduction
  2. Network Topology
  3. Hardware Connections
  4. TCP/IP Ports and Addresses
  5. Network Protocol Levels
  6. Data Link Layer and IEEE
  7. Network Protocol Categories
  8. Repeaters, Bridges, Routers
  9. ARP and RARP Address Translation
  10. Basic Addressing
  11. IP (Network)
  12. TCP (Transport)
  13. UDP (Transport)
  14. ICMP
  15. Hardware Cabling
  16. Wireless media
  17. Outside Connections
  18. Ethernet
  19. Token Ring
  20. ARCnet
  21. AppleTalk
  22. FDDI
  23. IPX/SPX
  24. NetBEUI
  25. AppleTalk
  26. SNA
  27. Others
  28. Simple Routing
  29. More Complex Routing
  30. IP Masquerading
  31. Firewalls
  32. Domain Name Service (DNS)
  33. Virtual Private Networking
  34. DHCP
  35. BOOTP
  36. RPC and NFS
  37. Broadcasting and Multicasting
  38. IGMP
  39. Dynamic Routing Protocols
  40. Simple Mail Transfer Protocol (SMTP)
  41. Simple Network Management Protocol
  42. Network Services
  43. Installing Drivers
  44. Network Operating Systems
  45. Applications
  46. Wide Area Networks
  47. Backing up the network
  48. Fault Tolerance
  49. Troubleshooting
  50. Commonly used Network Ports
  51. Networking Terms and Definitions
  52. Networking RFCs and Protocols
  53. Further Reading
  54. Credits

More Complex Networking Routing

Now let's modify the small network in the example in the previous section. The 192.168.1.x network is changed to 192.168.x.x and gateway B's address is changed to 192.168.10.1. All the netmasks on the computers on the 192.168.x.x network are modified to 255.255.0.0 to accommodate the change, except machine S3 which keeps the netmask 255.255.255.0 and changes its address to 192.168.10.3. This effectively puts S3 on a different network than S2 and S1, it no longer believes it can talk directly to them and must talk to gateway B to talk to them. It can't even talk to gateway A anymore since it can't address it directly. Machines S1, S2, and A are not on network 192.168.10.0, their addresses are 192.168.1.*. S1 and S2 can talk to S3, but S3 will not be able to respond unless it utilizes gateway B.

Small Network

Please be aware, in the example in the previous section, that gateway A was aware of gateway B. If it were not, no messages could have been transmitted from the internet to the 192.168.2.0 network. In this example, gateway A knows nothing about gateway B, and as far as it's concerned, the network 192,168.2.0 is part of 192.168.0.0 and there is no gateway between them. Gateway B, does know about gateway A and is using that gateway as its default gateway. Therefore if S1 and S2 use gateway A for their default gateway, they will not be able to talk to S4, 5, or 6 unless their routing table is modified. S1 and S2 will be able to talk to S3, however, assuming S3 is using gateway B.

Here is a listing of machine S1's routing table, using gateway A as default and no other routes.


DestinationGatewayGenmaskFlagsMetricRefUseIface
192.168.1.5*255.255.255.255UH000eth0
192.168.0.0*255.255.0.0U000eth0
127.0.0.0*255.0.0.0U000lo
default192.168.1.10.0.0.0UG000eth0


Here it is modified to let it use network 192.168.2.0.

DestinationGatewayGenmaskFlagsMetricRefUseIface
192.168.1.5*255.255.255.255UH000eth0
192.168.0.0*255.255.0.0U000eth0
192.168.2.0192.168.10.1255.255.255.0UG000eth0
192.168.2.0*255.255.255.0U000eth0
127.0.0.0*255.0.0.0U000lo
default192.168.1.10.0.0.0UG000eth0

It specifies the gateway B, 192,168.10.1 to be used if the destination is 192.168.2.x.

The figure below shows an ethernet network with bus topology excluding the hubs. It is a large Class A network with many subnetworks. The machines labeled A through D are routers or potential routers and each have two network interface cards(NIC). These machines may be called gateways since their function is to be a gate to another location. Each card has a valid address on its own network or subnetwork. The table below lists each gateway, and each NIC address and associated network.

Gateway eth0 eth0 networketh1eth1 network
A10.0.0.110.x.x.x164.25.74.131Internet
B10.0.0.210.x.x.x10.1.0.110.1.x.x.
C10.0.0.310.x.x.x10.2.0.110.2.x.x.
D10.0.0.410.x.x.x10.3.0.110.3.x.x.
E10.3.50.110.3.x.x10.3.100.110.3.100.x.
F10.1.0.210.1.x.x10.1.20.110.1.20.x.
G10.2.0.210.2.x.x192.168.1.1192.168.1.x.
H10.3.100.210.3.100.x10.3.150.110.3.150.x.
I10.3.150.210.3.150.x192.168.1.2192.168.1.x.


Small Network

In this figure, there are 9 gateways. which are labeled A through I. There are multiple paths between several networks. The possible paths between networks 10.1.100.x and 192.168.1.x can be through gateways E, D, C, then G (E-D-C-G) or through gateways H-I. The path from 10.3.100.x ot 10.1.20.x can be E-D-B-F or H-I-G-C-B-F. Obviously there are ways to set the routing paths up that may not be fully efficient. In this type of network, the administrator must give careful thought to the setup of the routing tables in their gateways. It would be easy to set up an infinite packet route loop in this network where some packets may go in circles from router to router. Here's how I would route for this network.

The below table lists each network and their default router.

NetworkDefault Router
10.3.100.xE
10.3.150.xH
192.168.1.xG
10.1.20.xF
10.1.x.xB
10.2.x.xC
10.3.x.xD
10.x.x.xA

The router, I, is not used as a default router for any network.

The table below lists an abbreviated route table for each gateway.

RouterDestinationGateway
A192.168.1.xC
10.1.x.xB
10.2.x.xC
10.3.x.xD
10.x.x.x10.0.0.1
defaultinternet
B10.1.20.xF
10.1.x.x10.1.0.1
defaultA
C192.168.1.xG
10.2.x.x10.2.0.1
defaultA
D10.3.150.xE
10.3.100.xE
10.3.x.x10.3.0.1
defaultA
E192.168.1.x *H
10.3.150.xH
10.3.100.x10.3.100.1
defaultD
F10.1.20.x10.1.20.1
defaultB
G10.3.100.x *I
192.168.1.x192.168.1.1
10.3.150.x *I
defaultC
H192.168.1.xI
10.3.100.x10.3.100.2
10.3.150.x10.3.150.1
defaultE
I10.3.100.xH
192.168.1.x192.168.1.2
10.3.150.x10.3.150.2
defaultG

The destinations with '*' indicate destinations that shorten the normal route path through network 10.3.150.x.

Also in this network since there are multiple possible paths, dynamic routing can be used to provide alternate routing, if one router goes down.