ALL ABOUT LAN

What is a LAN?

Okay, most of you already know what a LAN is but let’s give it a definition to make sure. We have to do this because, if you don’t know what a LAN is, you can’t understand what a VLAN is.

A LAN is a local area network and is defined as all devices in the same broadcast domain. If you remember, routers stop broadcasts, switches just forward them.
What is a VLAN?

As I said, a VLAN is a virtual LAN. In technical terms, a VLAN is a broadcast domain created by switches. Normally, it is a router creating that broadcast domain. With VLAN’s, a switch can create the broadcast domain.

This works by, you, the administrator, putting some switch ports in a VLAN other than 1, the default VLAN. All ports in a single VLAN are in a single broadcast domain.

Because switches can talk to each other, some ports on switch A can be in VLAN 10 and other ports on switch B can be in VLAN 10. Broadcasts between these devices will not be seen on any other port in any other VLAN, other than 10. However, these devices can all communicate because they are on the same VLAN. Without additional configuration, they would not be able to communicate with any other devices, not in their VLAN.
Are VLANs required?

It is important to point out that you don’t have to configure a VLAN until your network gets so large and has so much traffic that you need one. Many times, people are simply using VLAN’s because the network they are working on was already using them.

Another important fact is that, on a Cisco switch, VLAN’s are enabled by default and ALL devices are already in a VLAN. The VLAN that all devices are already in is VLAN 1. So, by default, you can just use all the ports on a switch and all devices will be able to talk to one another.
When do I need a VLAN?

You need to consider using VLAN’s in any of the following situations:

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You have more than 200 devices on your LAN
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You have a lot of broadcast traffic on your LAN
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Groups of users need more security or are being slowed down by too many broadcasts?
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Groups of users need to be on the same broadcast domain because they are running the same applications. An example would be a company that has VoIP phones. The users using the phone could be on a different VLAN, not with the regular users.
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Or, just to make a single switch into multiple virtual switches.

Why not just subnet my network?

A common question is why not just subnet the network instead of using VLAN’s? Each VLAN should be in its own subnet. The benefit that a VLAN provides over a subnetted network is that devices in different physical locations, not going back to the same router, can be on the same network. The limitation of subnetting a network with a router is that all devices on that subnet must be connected to the same switch and that switch must be connected to a port on the router.

With a VLAN, one device can be connected to one switch, another device can be connected to another switch, and those devices can still be on the same VLAN (broadcast domain).
How can devices on different VLAN’s communicate?

Devices on different VLAN’s can communicate with a router or a Layer 3 switch. As each VLAN is its own subnet, a router or Layer 3 switch must be used to route between the subnets.
What is a trunk port?

When there is a link between two switches or a router and a switch that carries the traffic of more than one VLAN, that port is a trunk port.

A trunk port must run a special trunking protocol. The protocol used would be Cisco’s proprietary Inter-switch link (ISL) or the IEEE standard 802.1q.
How do I create a VLAN?

Configuring VLAN’s can vary even between different models of Cisco switches. Your goals, no matter what the commands are, is to:

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Create the new VLAN’s
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Put each port in the proper VLAN

Let’s say we wanted to create VLAN’s 5 and 10. We want to put ports 2 & 3 in VLAN 5 (Marketing) and ports 4 and 5 in VLAN 10 (Human Resources). On a Cisco 2950 switch, here is how you would do it:

At this point, only ports 2 and 3 should be able to communicate with each other and ports 4 & 5 should be able to communicate. That is because each of these is in its own VLAN. For the device on port 2 to communicate with the device on port 4, you would have to configure a trunk port to a router so that it can strip off the VLAN information, route the packet, and add back the VLAN information.
What do VLAN’s offer?

VLAN’s offer higher performance for medium and large LAN’s because they limit broadcasts. As the amount of traffic and the number of devices grow, so does the number of broadcast packets. By using VLAN’s you are containing broadcasts.

VLAN’s also provide security because you are essentially putting one group of devices, in one VLAN, on their own network.
Article Summary

Here is what we have learned:

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A VLAN is a broadcast domain formed by switches
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Administrators must create the VLAN’s then assign what port goes in what VLAN, manually.
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VLAN’s provide better performance for medium and large LAN’s.
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All devices, by default, are in VLAN 1.
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A trunk port is a special port that runs ISL or 802.1q so that it can carry traffic from more than one VLAN.
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For devices in different VLAN’s to communicate, you must use a router of Layer 3 switch.

Home Network Setup Option 1: Everything hooked to a Hub or Switch

Pros

* Simple setup
* No server needed
* Configure each PC manually

Cons

* Maximum 4 PC's
* Fixed IP addresses for all 4 PC's
* Dial-in required per PC
* No Proxy
* Hub ports = the number of PC's + 1
* Only one PC at a time can be connected to the Internet

With this home network setup we connect all Ethernet cables (PC and ADSL) to a hub or switch. The hub or switch can be any model UTP based hub preferably with a uplink/MDI crossed connector. You will need the uplink connector to connect the ADSL connector. If your hub does not come with such a connector, you should buy or prepare a UTP crossed-over network cable and use that instead. Your hub should have at least the amount of PC's plus 1 (ADSL) UTP ports.

Layout:

Internet
|
|
Splitter
|
|
|
212.143.143.12 (Real IP from ISP - sample)
|
Alcatel modem
|
10.0.0.138
|
|
(Crossed-over cable if hub doesn't have an uplink port)
|
|
PC1-------------------Switching Hub-----------------------PC4
10.0.0.101 / \ 10.0.0.104
/ \
/ \
| |
| |
| |
| |
PC2 PC3
10.0.0.102 10.0.0.103

1. The ADSL cable goes to the splitter.
2. The ADSL modem is connected to the splitter.
3. The Ethernet cable from the modem either is connected to the uplink UTP connector of your hub, or - using a UTP crossed cable - to a regular UTP connector.
4. Each PC is connected to a regular UTP port of the hub/switch.
5. Each PC (maximum of 4!) should be configured as a ADSL client as described by your provider. Each PC should have an unique IP address.

Home Network Setup Option 2: Everything to the Hub or Switch - one of the PCs is a server

Pros

* Simple setup
* No manual PC configuration
* Almost no limit to the number of PC's (254)
* Dialup is done automatically
* Proxy (WinRoute or similar)

Cons

* Server software (NAT or Proxy) or Microsoft Internet Connection Sharing (ICS) required
* Configuring the server can be difficult
* Hub ports = number PC's + 1

This network setup is similar to variant 1 with one difference: we have a server which enables us to use a different IP range for PCs.

Layout:

Internet
|
|
Splitter
|
|
|
212.143.143.12 (Real IP from ISP - sample)
|
Alcatel modem
|
10.0.0.138
|
|
(Crossed-over cable if hub doesn't have an uplink port)
|
|
PC1-------------------Switching Hub-----------------PC4 (SERVER)
10.0.0.101 / \ 10.0.0.104
/ \
/ \
| |
| |
| |
| |
PC2 PC3
10.0.0.102 10.0.0.103

1. The ADSL cable goes to the splitter.
2. The ADSL modem is connected to the splitter.
3. The Ethernet cable from the modem either is connected to the uplink UTP connector of your hub, or - using a UTP crossed cable - to a regular UTP connector.
4. Each PC is connected to a regular UTP port of the hub/switch.
5. One of the PC's is to be configured as server either using software like WinRoute or Microsoft Internet Connection Sharing (Windows 2000, XP and ME have this standard included).

Home Network Setup Option 3: All PC's to a Hub or Switch - Server connected to ADSL

Pros

* Simple setup
* No manual configuration of PC's
* Almost unlimited number of PC's (254)
* Dialup done automatically
* Separation between the Internet and the LAN
* No uplink or crossed UTP cable required
* Hub ports = number PC's
* Proxy (WinRoute or similar)

Cons

* Server software (NAT or Proxy) or Microsoft Internet Connection Sharing (ICS) required
* Configuring the server can be difficult
* Hub ports = number PC's + 1
* Server needs additional network-card

Similar to option 1 and option 2 with the exception that the ADSL cable is connected to a server directly and from that server an second network connection is connected to the hub or switch. There are no limitations to the switch or hub used, this can even be a coax network if you would like so (this requires NO hub).

The ADSL network cable is connected to the server, which has 2 network-cards:

1. ADSL connection
2. LAN/hub connection

Layout:

Internet
|
|
Splitter
|
|
|
212.143.143.12 (Real IP from ISP - sample)
|
Alcatel modem
|
10.0.0.138
|
|
|
|
|
10.0.0.5
|
PC5 Server
|
192.168.0.5
|
|
|
|
|
PC1-------------------Switching Hub-----------------PC4
192.168.0.101 / \ 192.168.0.104
/ \
/ \
| |
| |
| |
| |
PC2 PC3
192.168.0.102 192.168.0.103

1. The ADSL cable goes to the splitter.
2. The ADSL modem is connected to the splitter.
3. The Ethernet cable from the modem is connected to the server.
4. The second network-card of the server is connected to the hub.
5. Each PC is connected to a regular UTP port of the hub/switch.
6. One of the PC's is to be configured as server either using software like WinRoute or Microsoft Internet Connection Sharing.

Home Network Setup Option 4: PC's and ADSL to Router/Switch

Pros

* Simple setup
* No manual configuration of PC's
* Almost unlimited number of PC's (254)
* Dialup done automatically
* Separation between the Internet and the LAN
* No uplink or crossed UTP cable required
* Hub ports = number PC's
* No server required

Cons

* Routers can be expensive
* Configuration of a router can be hard
* Not all ADSL routers support RAS with VPN/PPTP

The router/switch (like the LinkSys models) is both a "server" for DHCP, firewall, DNS etc. and a hub in one.

Layout:

Internet
|
|
Splitter
|
|
|
212.143.143.12 (Real IP from ISP - sample)
|
Alcatel modem
|
10.0.0.138
|
|
|
10.0.0.1
|
PC1------------------Router / Switch-----------------PC4
192.168.0.101 / \ 192.168.0.104
/ \
/ \
| |
| |
| |
| |
PC2 PC3
192.168.0.102 192.168.0.103

1. The ADSL cable goes to the splitter.
2. The ADSL modem is connected to the splitter.
3. The Ethernet cable from the modem is connected to the router/switch.
4. All PC's are connected to the router/switch.
5. All you need to do is to configure the router for Internet Sharing.

Note: Some ADSL modems, such as the Alcatel SpeedTouch Pro, can function as routers between the Internet and the LAN. Read Upgrade from Alcatel SpeedTouch Home to Pro on how to configure your modem. In that case you will no longer require a router on your network:

Layout:

Internet
|
|
Splitter
|
|
|
212.143.143.12 (Real IP from ISP - sample)
|
Alcatel modem (Configured as a router)
|
10.0.0.138
|
|
|
|
|
PC1-------------------Switching Hub-----------------------PC4
10.0.0.101 / \ 10.0.0.104
/ \
/ \
| |
| |
| |
| |
PC2 PC3
10.0.0.102 10.0.0.103

1. The ADSL cable goes to the splitter.
2. The ADSL modem is connected to the splitter.
3. The ADSL modem is configured as a router.
4. The Ethernet cable from the modem is connected to the switching hub.
5. All PC's are connected to the switching hub.



Remember these are just a few of the options available in a home network setup.