Network terminology explained

shumifan50

It seems a lot of people have trouble with setting up networks, so I will try to explain some terminology and their use.

IP address: This is like a street address and uniquely identifies a computer on the network.
Static IP address: A static IP address is assigned to a device when it is set up by the person installing the device. This IP address has to be allocated by a person that controls the network. In your own house it would be you and you can decide what IP address to assign to which device. However, you have to bear in mind that all devices that wish to talk to each other should be on the same subnet (explained below).
Dynamic IP address: A network can automatically control the assignment of IP addresses. This is done by a DHCP server. Each device on the network, that is setup for DHCP, will request an IP address when it is turned on and at intervals thereafter called lease times. This means that it is impossible to know the IP address of a given device at any moment in time without going to some trouble, and then it might change in the next second. This is not very useful when used with dreamboxes.
Subnet: Subnet is what it sounds like, it is a sub network or simplisticly a collection of IP addresses that can communicate with each other directly. The range of addresses is specified through a subnet mask, typically in the home environment it will be 255.255.255.0 where the 255 positions of all devices in the subnet have to match EXACTLY, whereas the '0' position has to be unique for each device. When you need more than 254 devices you can set up more complex subnet masks, but that is not needed here. Address 0 and 255 are reserved and cannot be used for devices.
Switch: A switch is a device that allows the connection of many devices to a physical network. In most homeuse cases it will merely provide a lot of sockets into which many devices can plugged. All devices on a switch need not have the same subnet - this means devices from multiple subnets can be plugged into the same switch, but only devices on the same subnet will be able to communicate with each other. Switches can sometimes have embedded firewalls but normally will not.
Bridge/Router:In the past a bridge had much less functionality than a router. These days they are the same in all but name as far as home users are concerned. A router is a device that allows communication between different subnets, a typical case is between a home network and the internet, but it could also be between multiple subnets in a single home network. The router will typically have an IP address on all networks that it routes between. So in the case of the internet connection, it will have an internet address as well as a home network address. All traffic between different subnets have to pass through a router. Routers typically, these days, have embedded firewalls and packet filtering capabilities.(see below).
DNS: This is a name service that provides a world wide lookup of URLs (like _www.goolge.com) to IP addresses. This is needed as all network traffic is based on IP addresses and not on names. So when you enter a URL the DNS server is contacted to translate the URL to an IP address and then the IP address is used for all communication.
Dynamic DNS or DDNS:
Most service providers dont give out static IP addresses. This makesit impossible to set up a web server or server of any description. The work around is to sign up to a DDNS provider like noip.org. These DDNS providers require that you run a little module on your device that keeps updating their servers with the latest IP address of your domain. They also disallow caching of your IP on any DNS servers so that each request to access your domain has to get the IP from their server. This way the DNS lookup of your IP from your domain name is always correct. Your dreambox has a builtin module to do the updates called inadyn. DDNS access is by the nature of the lookup much slower than when you have a static IP address.
Domain names:A domain name is the a name used to group computers together and has most signicance in how browsers allow navigation between different URLs. The whole process is too complicated to describe here. Domains are used to generate URLs that is used in DNS servers to do the IP lookup. A typical domain name will end in .com, .co.uk, .org, .edu, .tv etc etc. It allows addressing specific server seamlessly by using server.domainname.co.uk or the main entry to the domain as _www.domainame.co.uk, as an example.
Firewall:A firewall is a piece of hardware or software that allows you to set up rules as to what traffic you want to allow into and out of your network. The rules can be simple rules that merely block ports (like port 80 for webserver traffic), or much more complex by actually inspecting the payload of packets. They can also behave in 'stealth' mode by not responding to requests that violate access rules or they can just deny the access, meaning the violator knows the server is there even though it is not accessable - this might lead to more persistent attacks.
Port forwarding:If we have only one IP address on a network (like we typically have on the internet), and we have several devices on our home network, we need to tell the router which internal IP address provides the service for a given port on the internet. Lets say we have 2 servers (A and B) and we would like to provide 2 websites, but our service provider gave us only one internet IP address. In this case we can forward port 80 (webserver port normally) to server A and forward port 8080 (a port we picked for our second webser) to port 80 on server B. Now both servers can be accessed from the internet by using the same IP(domain name), but different ports. So the URLs would be something like this:
Server A: _http://www.mydomain.co.uk Note that I dont have to specify a port when I use http to port 80
Server B: _http://www.mydomain.co.uk:8080
Workstation setup:
The following is required when setting up a device on a network:
a. IP address. This can be DHCP or static (see above).
b. Subnet mask. Typically 255.255.255.0
c. Default gateway. This should point at a router to communicate with devices in a different subnet. In the case of the internet it will be the address of the internet router, which in the case of most ADSL modems is built into the the ADSL modem. If you dont wish to go outside your subnet, this entry is optional. In the case of dreamboxes we typically want to connect to the internet, so this entry should be setup to point at the ADSL or cable router.
d. DNS server(s): These are required and have to be set up if you wish to use URLs to access network resources rather than IP addresses. Using URLs is strongly recommended as a device will still be found, even when its IP changes.

The next post outlines an example setup.

shumifan50

First you decide on your subnet adress, in this example 192.168.1

Dreambox:
IP: 192.168.1.3 (static)
Subnet mask 255.255.255.0
Default gateway: 192.168.1.254 (must match IP of router below)
DNS: Has to be obtained from your service provider. Virgin would be 194.168.4.100 and 194.168.8.100. Notice it is on a different subnet.

Router:
IP: 192.168.1.254 (note this is the internal, home LAN IP and is static)
subnet mask: 255.255.255.0
DNS: as above
Internet IP: the router will get this from the service provider typically. Some service providers give static IPs that can be set up here.
Port forwarding:If you wish to make some services on your dreambox accessible from the internet, then you would have to forward the relevant port to the dreambox IP(192.168.1.3). For example if you would like to allow access to your dreambox web interface, then you would have to forward port 80 to 192.168.1.3:80. Some routers allow forwarding a port(like port 80) to a different port on the server.
Open Port In firewall:If you wish to allow access to a port on your dreambox and you have enabled a firewall in the router(strongly recommended), then you will have to unblock the port in the router.

PC:
IP: 192.168.1.2 (static)
Subnet: 255.255.255.0
gateway: 192.168.1.254 (must match router internal IP).
DNS: as for dreambox.

With the above setup all devices will be able to communicate with each other.

compufunk

Good work Shumifan. Your explanation should be of a lot of help to other members.

I'd like to add one point.

I've seen the term subnet being used a lot lately to describe what is in fact a network. There seems to be some confusion here. If someone goes reading up on Subnets, they will be reading up on something they don't really need to know, further complicating things for them.

Subneting is where a single network is slit into more than one network (sub networks). For example, your home class C network (192.168.1.0) could be split into 2 separate subnets by modifying your subnet mask to 255.255.255.128, or split into 4 using a subnet mask of 255.255.255.192 etc etc. Every time you do this, you are reducing the number of hosts you can have on each subnet by half -2.

These subnets would behave like separate networks meaning you'd need a bridge or router to communicate between them. Hosts on separate subnets can not directly communicate with each other.

I cant think of a reason why anyone would want to do this on a home network. It would normally only be done for large corporate, campus etc networks.

You can see how subnet masks and address ranges are calculated here _http://www.subnet-calculator.com/

shumifan50

How to use some of the basic utilities:

The following assumes you are in the console on the device for which you are testing the connectivity. This can be the physical console or a telnet session to the device.

1. To work out whether a device on the network can communicate with another device on the network, you can use the ping utility:
ping <ip address>
This will show "request timed out" or success.
However ping has limited value as some firewalls will disallow ICMP and therefore ping will not work.
2. A better way is to try and connect to a specific port (the one you need) by using telnet.
telnet <ip address> <port>
If the comms is set up correctly, then you will get a connection (with which you can do nothing) otherwise the connection will fail. This tests EXACTLY what you want to do. It can also be used to test the communication over the internet. When the connection works, expect to see garbage displayed on screen otherwise "connection failed".
3. To test DNS setup you can use ping <url> and it will display the IP it thinks is associated with the URL. The ping might still fail for various reasons, but if the IP is found it means the DNS is set up correctly.

Problems I have had in the past:
A. Ubuntu linux ships with IPv6 enabled, which, according to the Ubuntu documentation can cause problems, like connection failures. This can be disabled easily (from Ubuntu documentation):

1. IPv6 is supported by default in Ubuntu and can sometimes cause problems.
2. To disable it, open a Terminal (Applications → Accessories → Terminal) and type the command: gksudo gedit /etc/modprobe.d/aliases.
3. Find the line alias net-pf-10 ipv6 and change it to read alias net-pf-10 off.
4. Reboot Ubuntu.

B. If you have more than one router on your network, make sure you set the default gateway on a device to point at the router on which you are doing portforwarding to that device. If this is set up incorrectly then it will be possible to ping the device on the internal network, but not from the router. I did this incorrectly and it took me a while to find the problem.

C. If you do not use static IP addresses for all your devices on your network, then make sure that you set up the range of IP addresses in your DHCP server (normally the router) that DHCP can issue. Make sure that none of your static IP addresses fall with in this range.

D. SECURITY: Make sure that you change your router username/password as soon as possible - DO NOT LEAVE IT AS DEFAULT. This also applies to your dreambox, especially if you expose the admin or web interface to the internet.

aileen

Is there one like this relating to sat TV items such as 2-4-4 and BISS etc ?

compufunk

Take a look at the document regarding digital transmissions posted by here by rolfw.

PDF file explaining Digital transmissions

And here for encryption systems ....
_http://www.duwgati.com/uk/index.htm