Confusion is very common when it comes to understand the differences between dynamic and static/fixed/public IPs and its functioning or even how many IPs you should need to contract for a specific solution.

An IP address is a 32 bit number displaced in 4 parts, so 4 digits from 0 to 255 separated by dots. For example 196.122.0.255 would be one. Each of the 4 parts is made of 8 binary digits. The maximum number we can get in each part is 255 for a simple reason, with 8 bits it is the maximum number we can generate. We will not here explain how binary works, but it is useful to have some basics to fully understand the IP address concept. By now, it will be enough knowing that we can have a limited number of unique IPs by having 32 binary digits, IP are then not unlimited or infinite. A fixed IP or public IP would be defined as the a unique number assigned by an ISP to be its permanent address on the internet.

Why there are different types of IP addresses

All computers need an IP address to be able to communicate between them just as each telephone needs a specific and unique number. The Internet architects were not expecting such a high need of IP addresses being this the reason why it is not possible to give a permanent IP address to everybody as we would just run out of IPs. What ISPs do is having a pool of fixed IPs that is automatically assigned in a per session basis to their customers. This way, with a limited number of IPs, they can serve a much higher number of demands. When a computer has no fixed IP address assigned, it must then use a dynamic IP by requesting it to the ISP, this is called DHCP (Dynamic Host Configuration Protocol). Once the user disconnects from the Internet, the IP address goes back into the IP address pool so it can be assigned to another user, and that is why this type of IPs are called dynamic. If the same user later goes online again most provably the IP address assigned will be different. It is not possible to maintain the same IP address for a long period of time if we use dynamic IP. If for any reason we need a permanent IP we will either require a Dynamic DNS service or contract a permanent IP to the operator. The cost of a fixed IP basically depends on the technology and country but as a standard it would go from 5€ to 25€/month.

We have several options once we contract an internet access. The best will always be contracting your own static IP, but as this option will not be free of cost it can be a good idea to get a service called Dynamic DNS. If you register into a DNS service when you connect to the Internet using a dynamic IP address, your computer will contact the DNS service and let it know what IP address has been assigned to you from the IP pool. The DNS server then forwards the correct address to the requesting DHCP computer. Other computers will be able to always find you even if your IP has changed for being dynamic. The third and last option is to have a dynamic IP being this the most common solution for residential users.

Typical options. 1, 4 and 8 IP range.

When it comes to requesting IP addresses to a provider we will normally have different options available.

-1 single IP /32 IP@ (called a slash 32 IP address). Only one equipment needs to be remotely accessible or monitored. In our case we do this when we need to monitor an international internet access with only one single equipment laying behind. It is also possible to have a modem-router configured in bridge mode letting this way the equipment connected behind to get the single fixed IP.
-4 IP range /30 IP@. As we are getting 4 IPs and 2 will not be usable by definition we have 2 IP free for us to use. Normally one will be assigned to the ISPs router and you will have 1 left free to use. This is the most common business type solution as it is typical to install an IPsec router or firewall behind the DSL modem-router. We will be able to monitor both routers separately.
Example:

196.122.0.255 if we convert the 255 in binary code we get

196.122.0.[111111/11]

 

As an IP has 32 bits a /30 IP will give us 32-30= 2 bits free. We have 2 free binary digits to play with which can create 4 different IPs.

00 -> Network IP address -> (11111100) -> 196.122.0.252 (Not usable)

01 -> IP address normally assigned to the ISP router -> (11111101) -> 196.122.0.253

10 -> Free IP address for us to use. We can connect a Firewall, IPsec router, etc., taking this customer usable IP address. -> (11111110) -> 196.122.0.254

11 -> Broadcast IP address -> (11111111) -> 196.122.0.255 (Not usable)

 

So we when we are told that we will be getting a 196.122.0.252/30 IP address, from this information we should be able to see that our IP range will be 196.122.0.252 – 196.122.0.255 of IPs for us to use and our first usable IP address will be 196.122.0.253.

-8 IP range /29 IP@ As we are getting 8 IPs and 2 will not be usable by definition we have 6 IPs free for me to use. Normally one will be assigned to the ISPs router and you will have 5 left free to use.

 

Example:

196.122.0.255 if we convert the 255 in binary code we get

196.122.0.[11111/111]

 

As an IP has 32 bits a /29 IP will give us 32-29= 3 bits free. We have 3 free binary digits to play with which can create 8 different IPs.

000 -> Network IP address -> (11111000) -> 196.122.0.248 (Not usable)

001 -> IP address normally assigned to the ISP router -> (11111001) -> 196.122.0.249 –

010 -> Free IP address for us to use. We can connect a Firewall, IPsec router, etc., taking this customer usable IP address -> (11111010) -> 196.122.0.250

011 -> Free IP address -> (11111011) -> 196.122.0.251

100 -> Free IP address -> (11111100) -> 196.122.0.252

101 -> Free IP address -> (11111101) -> 196.122.0.253

110 -> Free IP address -> (11111110) -> 196.122.0.254

111 -> Broadcast IP address -> (11111111) -> 196.122.0.255 (Not usable)

 

So if we are just given this information: 196.122.0.248/29 IP address, just with this we should be able to see that we will be assigned the range going from IP 196.122.0.248 to 196.122.0.255. As first and last IPs are not usable by definition, once the ISP connects the router you should ping the first usable IP address which is 196.122.0.249 to see it responding once the line and router are up and running.

 

IPv6

IPv6, also been called “IPng” (IP Next Generation), lengthens IP addresses from actual 32 up to 128 bits increasing the number of available IP addresses. This will most probably make static IP addresses easier and less expensive to obtain and maintain. Dynamic IPs should end up disappearing as we will be able to have enough unique IPs for everybody making address pools nonsense.