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»Cisco Forum FAQ »Quick and Easy Subnetting on Routing, Switching and Network Design Relationship

Let's say we have the following network

Router 4
| | |
+----------------+ | +-------------------+
| | |
Router 1 Router 2 Router 3
| | | | | |
1st | 3rd | 4th 5th network
2nd network Switch
| | |
| | | | | | | | |
Server | Server Server | Server Printer | Printer
Server Server Printer

Let's say that there is broadcast network in use here for all networks within the organization. Within a broadcast network, hosts intra-communicate by using Layer 2 mechanism (switching). To interconnect hosts from one network to another (or to the Internet), there will be Layer 3 mechanism (routing).

In routing technology, then there will be gateway within each network. Hosts within each network are considered "dumb device" that has no knowledge of routing. Hosts only have Layer-2 knowledge (switching) to intra-communicate with other hosts within the same network. Hosts then will rely on gateway to handle the routing. This is where the routers are needed in the organization to provide the routing, which is the network inter-communication and access to the Internet.

Choosing Gateway for specific network

When configuring host network info, you may wonder which device or which IP address to use as network gateway. Referring to the organization network design, here are gateways for each network.

1st network gateway : Router 1
2nd network gateway : Router 1
3rd network gateway : Router 1
Server farm gateway : Router 2
4th network gateway : Router 3
5th network gateway : Router 3
Router 1, 2, 3 gateway: Router 4
Router 4 gateway : ISP device

You may wonder what consideration used to choose specific device as gateway of specific network. Following questions may arise.

* Can we use Router 4 as the 1st network gateway?
* Can we use Router 3 as the 2nd network gateway?
* How about using the ISP device as Server Farm gateway?

Earlier, we decided to have independent networks within the organization. With independent network, it means that you segment the organization to multiple smaller networks. Each network will have dedicated subnet. There is routing to interconnect smaller networks and to provide Internet access. The smaller networks are the 1st to 5th network, Server Farm, and Between Routers.

Let's look at the 1st network. Note that hosts within this network are the "dumb device" (i.e. servers, workstations, printers). As mentioned, these dumb devices use switching to intra-communicate.

To communicate with other hosts at different network or at the Internet, the 1st network hosts rely on gateway which then will do routing. Since the 1st network hosts are only capable of Layer-2 communication technique, the network gateway must be capable of doing the same in addition of routing capability.

Keep in mind that the Layer-2 communication only takes place within the 1st broadcast network. Therefore the 1st network gateway must be within the same broadcast network with other 1st network hosts in order to perform the Layer-2 communication. Once there are traffic outbound from the 1st network to other network or vice versa (the inbound), the gateway will route traffic between the one network and others.

Referring to the network design, Router 1 is within the same broadcast network with 1st network hosts. Therefore it is logical choice to use Router 1 as the 1st network gateway. Similar concept applies to 2nd to 5th networks.

Choosing Gateway for the Between Routers network

Now let's review the Between Router network. This network is also broadcast network. The network hosts are Router 1 to 4.

Since they are all routers, they are all able to perform routing. Which router to choose as gateway then?

Note that the gateway concept is to provide last resort of unknown or undefined network reachability. The objective of the organization routing design is to provide connectivity among hosts (dumb devices) within the organization and between hosts and Internet.

Notice that 1st to 3rd networks are behind Router 1, Server Farm network is behind Router 2, and 4th to 5th networks are behind Router 3. The Internet (ISP) is in front of Router 4.

Let's look at Router 1. From Router 1's perspective, it will use Router 2 to reach Server Farm. Router 1 will use Router 3 to reach 4th and 5th networks. Router 1 will use Router 4 to reach the Internet.

Similar perspective occurs at Router 2. Router 2 will use Router 1 to reach 1st to 3rd networks. Router 2 will use Router 3 to reach 4th and 5th networks. Router 2 will use Router 4 to reach the Internet.

In the organization, the defined or known networks are the 1st to 5th networks and Server Farm. Internet is considered undefined or "miscellaneous" networks from the organization's perspective. Therefore to reach the Internet, Between Routers network uses Router 4 as the network gateway.

Choosing Gateway for the Internet

There will be similar concept as the Between Routers network to apply to the Internet network. From Router 4's perspective, the Internet is in front of ISP device. From ISP perspective, the entire organization is behind the Router 4.

For the entire organization to reach the Internet, Router 4 will use ISP device as the gateway. Similarly, ISP device will use Router 4 to reach the entire organization.

Choosing the Gateway IP Address

Let's say that the entire network is using subnet to support connectivity. The ISP assigns specific subnet to provide the organization Internet connectivity.

Let's say the followings are the subnet assignments

1st network: ( -
2nd network: ( -
3rd network: ( -
4th network: ( -
5th network: ( -
Server Farm: ( -

Between Routers

Router 4 - ISP Device

Within the organization, you as the network designer have full access to determine which IP addresses are for hosts and which are for gateways. Let's say you have the followings as the "dumb device" network gateways.

1st network:
2nd network:
3rd network:
4th network:
5th network:
Server Farm:

Let's say that Router 1 to Router 3 have four independent interfaces where Router 4 has two interfaces. These interfaces will handle the routing for each network and also serve as gateway for specific network.

Followings are the interface IP address assignment.

Router 1

1st interface:
2nd interface:
3rd interface:
4th interface:

Router 2

1st interface:
2nd interface:

Router 3

1st interface:
2nd interface:
3rd interface:

Router 4

1st interface:

Let's say the ISP decides to use as their device that provide direct connection to the organization. Therefore you have the following IP address assignment.

Router 4

2nd interface:

With these IP address assignments, following are the gateways for each respective network.

Host network

1st network: (Router 1)
2nd network: (Router 1)
3rd network: (Router 1)
4th network: (Router 3)
5th network: (Router 3)
Server Farm: (Router 2)

Between Routers network : (Router 1) (Router 1) (Router 3) (Router 2)
The Internet : (Router 4)

Router 4 - ISP Device

The Internet : (ISP Device) (Router 4)

Some discussions

»[CCNA] How to calculate the next hop
»[HELP] ip route Null0 255

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by aryoba See Profile
last modified: 2011-10-18 07:03:20