Designing IP Address

How to Design the IP address for Interconnected Sites that Is Sometimes Complicated for some System Administrator?

One of the tasks we need to do in designing the network infrastructure is designing the IP address for each of the network sites and the interconnected sites across WAN. The need of good designing of the IP address is the fact that:

• Each physical network segment requires an address unique to the network
• Each host on a network requires an address that is unique to the network segment
• IP addresses are made up of the network ID and the host ID
• Address class and the subnet mask determine how much of the address makes up each ID

To be able to communicate on a private TCP/IP network or on the public Internet, each host on the network must be identified by a 32-bit IP address. These IP addresses can be grouped into two categories: Public IP addresses and private IP addresses.

1. Public addresses are the globally unique IP addresses that are connected to the Internet. To allow networks to obtain unique addresses for the Internet, the Internet Assigned Numbers Authority (IANA) divides up the non-reserved portion of the IP address space and delegates responsibility for address allocation to a number of regional registries throughout the world. These registries include Asia-Pacific Network Information Center (APNIC), American Registry for Internet Numbers (ARIN), and Réseaux IP Européens (RIPE NCC).

2. Private addresses are confined to specific ranges that can be used by any private network but that cannot be seen on the public Internet. The IANA has reserved a certain number of IP addresses that are never used on the global Internet. These private IP addresses are used for hosts that require IP connectivity but that do not need to be seen on the public network. The following table is the Private IP address range in each class of the IP address.

Case Study

In the previous scenario about network infrastructure that consists of three remote sites: Guinea Smelter (around 200 hosts); Brazil main site (around 1,000); and US office (around 400 hosts).

Desiging IP Address

Assume your IT Director gives you the IP address between 192.168.100.1 until 192.168.107.254 that you need to deploy to the three sites. How can you do that? Firstly we need to examine the IP need for the above three sites:

1. Guinea Smelter needs at least 200 hosts at present, but prediction for the next 5 years time the site will need the addition of about 200 more hosts. You need to allocate about 400 hosts for Guinea Smelter.

2. The headquarter office in US needs not more than 400 hosts including the prediction for the next 5 years time.

3. The Brazil main site needs 1,000 hosts including prediction in the next 5 years time.

We can see that each site needs more than one subnet in Class C IP address. It means that you need to combine two Class C subnets for both Guinea and US office that each needs around 400 IP addresses. While for the Brazil main site you need to combine 4 Class C subnets to yields around 1,000 hosts.

Default subnet for Class C address is 255.255.255.0. Let’s have a look at the following table; each of the networks address in this Class C IP address has a maximum of 254 hosts (8 bits for host, 2^8 – 2 = 254 hosts). To yield 400 hosts, you need 9 bits for hosts (2^9 – 2 = 510 hosts); while to yield 1,000 hosts you need 10 bits for hosts (2^10 – 2 = 1010 hosts).

Have a look at the fourth column in the above table that represents the binary form of the third octet (100 – 107) in the IP address. If you would like to yield 254 IP that will not be a big deal, you can use the default Class C address and the default Class C

subnet mask = 255.255.255.0

The default subnet mask 255.255.255.0 can be written with the notation /24 (means that the number of bit “1″ for subnet mask is 24), for example:

192.168.100.0/24; 192.168.101.0/24; 192.168.103/24 and so on

IP Address Design - 254 Hosts

The need of 400 hosts for Guinea Smelter and US office

To obtain the number of hosts up to 510 IP, you need to use the last 9 bit for the host and the first 23 bit for the network address. In this Class C network, you have to borrow 1 bit from the network address to get 9 bit for the hosts. To simplify the calculation you need 2 pair of network address Class C:

(192.168.100.0 and 192.168.101.0)

(192.168.102.0 and 192.168.103.0)

(192.168.104.0 and 192.168.105.0)

(192.168.106.0 and 192.168.107.0)

With 23 bit for the network address, means that the subnet mask is 255.255.254.0 or commonly written as /23 notation for the subnet mask. Remember the third octet is 254 (1111 1110) not 255 (1111 1111) because one bit is borrowed by the host.

Back to the scenario, we can decide now which IP address ranges both for Guinea Smelter (400 IP) and US office (400 IP):

• For Guinea Smelter the network address is 192.168.100.0/23
• And for US office the network address is 192.168.102.0/23.

IP Address Design - 510 Hosts

The need of 1,000 hosts for Brazil main site

To obtain the IP address for 1,000 hosts you need to use 10 bits (2^10 – 2 = 1,010 hosts) for the hosts and 22 bits for the network. The number of “1″ bit for your subnet mask is 22, so you need to use the subnet mask 255.255.252.0. Note that the third octet is 252 (1111 1100) because 2 bit is used for hosts.

IP Address Design 1010 hosts

To simplify the calculation you need to combine four Class C network addresses to yield 1,000 hosts. As we know that the first two network addresses have been used by Guinea Smelter and the next 2 network addresses has been used by US Office. The first four network address has been used so the rest 4 network address can be used for Brazil main site.

192.168.104.0 192.168.105.0 192.168.106.0 and 192.168.107.0

Or you can simplify by writing this as 192.168.104.0/22 with the subnet mask 255.255.252.0

The private address for the three sites has been designed. You can allocate the IP addresses both for the wired network clients and for the wireless clients from the DHCP server, as well as the computers with the wireless adapters (such as USB wireless adapters) that connect to the wireless access point. The next thing you need to design is the Public IP address for each of the end-point to the Internet for WAN interconnection.

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