IP addresses, ports and DNS: A quick introduction, just add wine.

So, you want to know more about IP addresses, ports, and DNS... Fortunately, it's not a combination of intellectual property, port and other drinks, and does not involve a Director of Nursing Services (though you may feel like you need a combination of these things once finished).

What is DNS?

Firstly, let's take a look at DNS. DNS involves many different elements including root servers, TLD servers, registrars, domain name services, website servers and e-mail servers. If you would like to know how DNS works, DNSSecrets provides a great infogram that simply explains how DNS works with plenty of helpful information and facts for unraveling the complexities of DNS.

What is an IP address?

Every device that is directly connected to the internet has a "public" IP address. Public IP addresses are used by servers on the internet, including servers for web sites, DNS servers, network routers or any computer connected directly to the Internet via a modem. Another way to approach the idea is that a public IP address is a completely unique address that is accessible from any other public IP address.

IP Version 4 addresses (the current standard) are comprised of 4 sets of digits between 0 and 255 which are separated by periods. For example, the IP address of www.racs.com.au is 54.252.148.183.

Although we use domain names to access things such as websites, nothing on the internet can be directly accessed by domain name (i.e.: www.racs.com.au), it can only be truly accessed by IP address. Considering that using IP addresses to access websites would be a real pain when you needed to access a website, the Domain Name Server system was invented, which converts a domain name to an IP address. For example, www.racs.com.au is converted to 54.252.148.183 in the background.

The conversion happens when you type www.google.com.au into Chrome, Firefox, Safari, or Internet Explorer's address bar. In the background, your computer asks your local DNS server what is the IP address for the domain name www.google.com.au. The server then replies with an IP address, taking you to the desired web site.

What are ports?

Now that we have covered IP addresses and DNS servers, what in the world are ports? Well, the way that I think of ports is that they are like a block of single bedroom units. The IP address is the street number, and the port number is the unit number.

Fact: Every IP address has 65535 ports that are usable. You may not realise it, but you use ports every day you're on the internet!

An example of using ports is accessing this website (the RACS blog). All public web servers always use port 80 for unsecured web traffic, so when you type www.racs.com.au into a web browser, it is actually accessing the IP address 54.252.148.183 : port 80 (or abbreviated to 54.252.148.183:80). If you were accessing a secure site, it would use port 443.

With single bedroom units, there is only ever one family or person staying in them, though many visitors may come and go, the same person stays in the unit. In the same way, ports only have one service attached to them, though traffic can come from more than one place to this port, the same service will be at that port (there are exceptions to this for higher numbered ports, but this is not essential).

Now what is the service that greets the traffic that comes to the port? Well, that all depends on what the IT Administrators have configured on that port. As mentioned, port 80 is a standard port for unsecured web traffic and port 443 is for secured web traffic.

That all sounds good in theory, what about a real life example?

Let's say that Joe wants to access his computer at work from home. How can we do this? At Joe's work, his computer is connected to the work network with the IP address of 192.168.0.25 (not public), and on the work network there is also a router with a public IP address of 1.2.3.4. Microsoft run the Remote Desktop service on port 3389 (don't ask why they use this exact port, they just do).

If we add a rule to the router on the work network for any traffic that comes to the public IP address 1.2.3.4 port 3355 (fictional port number for Joe, you can pick your own), it forwards on this traffic to the private IP 192.168.0.25:3389. (Remember the 3389 is the port for Microsoft Remote Desktop service). This means that when we try to access port 1.2.3.4:3355 from the internet, it will be connected to Joe's Remote Desktop service on his computer inside the network.

Now if Gary saw Joe's set-up and wanted the same thing, we can add another rule in the router for Gary's computer, but we can not use port 3355 because Joe's computer is already on that port. We might use port 3356.

For some examples on setting up port forwarding on Mikrotik routers as per the above example, we have an upcoming blog article about it. Alternatively, you can log a support ticket and we can help you get organised.

David Oates 30-Apr-2013 0 Comments
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