IP Address Plan
From Best Practices
Contents |
Introduction
Addressing concept describes the way the IP addresses are allocated to the different location and subnets of the network. A careful address plan helps in reducing the size of the routing tables therefore the efficiency and performances of the network.
The IP address plan also depends on the way the network design is organized and how your VLANs are structured. However, there are some bests practices which can help to create a clean IP address plan.
General guidelines
There are four main steps in organizing subnets for creating a clean IP address plan:
- Defining the type of networks you need.
- Defining the size of your subnets
- How you distribute subnets
- Defining how you allocate addresses in subnets
These three steps will be described more in detail in the following chapters.
Type of networks
If you have more that one subnet, you will have to define subnet types. For example, you might have server subnets, users subnets, firewall subnets, voice or media gateways subnets, network management subnets, etc. It is important that you enumerate all the subnet types at this step. Don't hesitate to write everything you have in mind, and remove redundancies after. Most of the time, your subnet types are related to your VLANs.
Size of subnets
For each subnet types you defined above, enumerate the different size you might need. The size of a subnet is defined by its netmask. Therefore you have to define subnet size in power. A table of these power is given there.
Note
Experience demonstrated that subnets should never be larger than 256 addresses (netmask of 255.255.255.0).
Distribution of subnets
The distribution of subnets has an important impact on the efficienciy of routing protocols. A careful alocation will lead to a good route summarization, which will help routersd to route efficiently in keeping routing tables as small as possible. In the counter part, a bad subnet definition might lead to very large routing tables, which could affetc the efficiency of the routers.
IP Address allocation
The IP address allocation is the way you distribute addresses inside a subnet. It depends on the subnet type, the type of devices you have on the subnet, etc.
Example of IP address plan
The tables below gives an example of IP address allocation which is comming from a running network. The addresses are allocated in the following way, including their netmask:
| Network type | Max amount of IP addresses | Netmask |
| 1 | 14 | 255.255.255.240 |
| 2 | 30 | 255.255.255.224 |
| 3 | 62 | 255.255.255.192 |
| 4 | 126 | 255.255.255.128 |
| 5 | 254 | 255.255.255.0 |
NA stands for network address, and max stands for the maximum amount of IP addresses of the subnet (table above).
Data subnet with 16 addresses
Subnet with 16 addresses, including the network and the broadcast address.
| Address | Usage |
| NA + 1 - 2 | Router |
| NA + 3 | Switch |
| NA + 4 | Server |
| NA + 5 - 6 | Printer |
| NA + 7 – 13 | PC (Dynamic) |
| NA + 14 | Reserve (Test) |
Data subnet with 32 addresses
| Address | Usage |
| NA + 1 - 4 | Router |
| NA + 5 - 6 | Switch |
| NA + 7 | Server |
| NA + 8 - 12 | Printer |
| NA + 13 - 29 | PC (Dynamic) |
| NA + 30 | Reserve (Test) |
Data subnet with 64 addresses
| Address | Usage |
| NA + 1 - 6 | Router |
| NA + 7 - 9 | Switch |
| NA + 10 - 11 | Server |
| NA + 12 - 19 | Printer |
| NA + 20 – 61 | PC (Dynamic) |
| NA + 62 | Reserve (Test) |
Data subnet with 128 addresses
| Address | Usage |
| NA + 1 - 6 | Router |
| NA + 7 - 15 | Switch |
| NA + 15 - 20 | Server |
| NA + 21 - 30 | Printer |
| NA + 31 - 125 | PC (Dynamic) |
| NA + 126 | Reserve (Test) |
Data subnet with 256 addresses
| Address | Usage |
| NA + 1 - 6 | Router |
| NA + 7 - 15 | Switch |
| NA + 15 - 20 | Server |
| NA + 20 - 40 | Printer |
| NA + 31 - 253 | PC (Dynamic) |
| NA + 254 | Reserve (Test) |
Subnets with more than 256 addresses
Subnets with more than 256 IP addresses (a C class) are strongly discouraged. Bests practice shown that they should be avoided as they create more problems than they solve.
Example
Let take the C class subnet (with 254 usable addresses) 192.168.1.0. NA is 192.168.1.0, max is 192.168.254. The IP address allocation is then:
| Address | Usage |
| 192.168.1.1 | Router |
| 192.168.1.2 | Router |
| 192.168.1.7 | Switch |
| 192.168.1.8 | Switch |
| 192.168.1.15 | Server |
| 192.168.1.16 | Server |
| 192.168.1.17 | Server |
| 192.168.1.20 | Printer |
| 192.168.1.21 | Printer |
| 192.168.1.22 | Printer |
| 192.168.1.23 | Printer |
| 192.168.1.31 | PC (Dynamic) |
| 192.168.1.32 | PC (Dynamic) |
| 192.168.1.33 | PC (Dynamic) |
| ... | ... |
| 192.168.1.253 | PC (Dynamic) |
| 192.168.1.254 | Reserve (Test) |
