Private and Public IP Addresses

What are Private and Public IP Addresses

Internet Protocol (IP) addresses are usually of two types: Public and Private. If you have ever wondered to know what is the difference between a public and a private IP address, then you are at the right place. In this post I will try to explain the difference between a public and a private IP addres in layman’s terms so that it becomes simple and easy to understand.

What are Public IP Addresses?

A public IP address is assigned to every computer that connects to the Internet where each IP is unique. Hence there cannot exist two computers with the same public IP address all over the Internet. This addressing scheme makes it possible for the computers to “find each other” online and exchange information. User has no control over the IP address (public) that is assigned to the computer. The public IP address is assigned to the computer by the Internet Service Provider as soon as the computer is connected to the Internet gateway.

A public IP address can be either static or dynamic. A static public IP address does not change and is used primarily for hosting webpages or services on the Internet. On the other hand a dynamic public IP address is chosen from a pool of available addresses and changes each time one connects to the Internet. Most Internet users will only have a dynamic IP assigned to their computer which goes off when the computer is disconnected from the Internet. Thus when it is re-connected it gets a new IP.

You can check your public IP address by visiting www.whatismyip.com

What are Private IP Addresses?

An IP address is considered private if the IP number falls within one of the IP address ranges reserved for private networks such as a Local Area Network (LAN). The Internet Assigned Numbers Authority (IANA) has reserved the following three blocks of the IP address space for private networks (local networks):

10.0.0.0 – 10.255.255.255 (Total Addresses: 16,777,216)

172.16.0.0 – 172.31.255.255 (Total Addresses: 1,048,576)

192.168.0.0 – 192.168.255.255 (Total Addresses: 65,536)

Private IP addresses are used for numbering the computers in a private network including home, school and business LANs in airports and hotels which makes it possible for the computers in the network to communicate with each other. Say for example, if a network X consists of 10 computers each of them can be given an IP starting from 192.168.1.1 to 192.168.1.10. Unlike the public IP, the administrator of the private network is free to assign an IP address of his own choice (provided the IP number falls in the private IP address range as mentioned above).

Devices with private IP addresses cannot connect directly to the Internet. Likewise, computers outside the local network cannot connect directly to a device with a private IP. It is possible to interconnect two private networks with the help of a router or a similar device that supports Network Address Translation.

If the private network is connected to the Internet (through an Internet connection via ISP) then each computer will have a private IP as well as a public IP. Private IP is used for communication within the network where as the public IP is used for communication over the Internet. Most Internet users with a DSL/ADSL connection will have both a private as well as a public IP.

You can know your private IP by typing ipconfig command in the command prompt. The number that you see against “IPV4 Address:” is your private IP which in most cases will be 192.168.1.1 or 192.168.1.2. Unlike the public IP, private IP addresses are always static in nature.

Unlike what most people assume, a private IP is neither the one which is impossible to trace (just like the private telephone number) nor the one reserved for stealth Internet usage. In reality there is no public IP address that is impossible to trace since the protocol itself is designed for transparency.

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How Firewall Works

If you have been using the Internet for any length of time, and especially if you work at a larger company and browse the Web while you are at work, you have probably heard the term firewall used. For example, you often hear people in companies say things like, "I can't use that site because they won't let it through the firewall."

How Firewall Works ?

Firewalls are basically a barrier between your computer and the internet. A firewall can be simply compared to security guard who stands at the entrance of your house and filters the

visitors coming to your place. He may allow some visitors to enter while denying others whom he suspects of being intruders. Similarly firewall is a software program or a hardware device that filters the information coming through the internet in form of packets to your personal computer or a computer network.

Firewalls may decide to allow or block network traffic between devices based on the rules that are pre-configured or set by the firewall administrator. Most personal firewalls such as Windows firewall operate on a set of pre-configured rules that are most suitable under normal circumstances so that the user need not worry much about configuring the firewall.

Personal firewalls are easy to install and use and hence preferred by end-users for use on their personal computers. However large networks and companies prefer those firewalls that have plenty of options to configure so as to meet their customized needs. For example, a company may set up different firewall rules for FTP servers, Telnet servers and Web servers. In addition the company can even control how the employees connect to the Internet by blocking access to certain websites or restricting the transfer of files to other networks. Thus in addition to security, a firewall can give the company a tremendous control over how people use the network.

Firewalls use one or more of the following methods to control the incoming and outgoing traffic in a network:

1. Packet Filtering: In this method packets (small chunks of data) are analyzed against a set of filters. Packet filters has a set of rules that come with accept and deny actions which are pre-configured or can be configured manually by the firewall administrator. If the packet manages to make it through these filters then it is allowed to reach the destination; otherwise it is discarded.

2. Stateful Inspection: This is a newer method that doesn’t analyze the contents of the packets. Instead it compares certain key aspects of each packet to a database of trusted source. Both incoming and outgoing packets are compared against this database and if the comparison yields a reasonable match, then the packets are allowed to travel further. Otherwise they are discarded.

Firewall Configuration

Firewalls can be configured by adding one or more filters based on several conditions as mentioned below:

1. IP addresses
 In any case if an IP address outside the network is said to be unfavorable, then it is possible to set filter to block all the traffic to and from that IP address. For example, if a certain IP address is found to be making too many connections to a server, the administrator may decide to block traffic from this IP using the firewall.

2.Domain names
Since it is difficult to remember the IP addresses, it is an easier and smarter way to configure the firewalls by adding filters based on domain names. By setting up a domain filter, a company may decide to block all access to certain domain names, or may provide access only to a list of selected domain names.

3. Ports/Protocols
Every service running on a server is made available to the Internet using numbered ports, one for each service. In simple words, ports can be compared to virtual doors of the server through which services are made available. For example, if a server is running a Web (HTTP) service then it will be typically available on port 80. In order to avail this service, the client needs to connect to the server via port 80. Similarly different services such as Telnet (Port 23), FTP (port 21) and SMTP (port 25) services may be running on the server. If the services are intended for the public, they are usually kept open. Otherwise they are blocked using the firewall so as to prevent intruders from using the open ports for making unauthorized connections.

4. Specific words/Phrases
 A firewall can be configured to filter one or more specific words or phrases so that, both the incoming and outgoing packets are scanned for the words in the filter. For example, you may set up a firewall rule to filter any packet that contains an offensive term or a phrase that you may decide to block from entering or leaving your network.

Hardware vs Software Firewalls

Hardware firewalls provide higher level of security and hence preferred for servers where security has the top most priority whereas, the software firewalls are less expensive and are most preferred in home computers and laptops. Hardware firewalls usually come as an in-built unit of a router and provide maximum security as it filters each packet in the hardware level itself even before it manages to enter your computer. A good example is the Linksys Cable/DSL router.

Why Firewall ?

Firewalls provide security over a number of online threats such as Remote login, Trojan backdoors, Session hijacking, DOS & DDOS attacks, viruses, cookie stealing and many more. The effectiveness of the security depends on the way you configure the firewall and how you set up the filter rules. However major threats such as DOS and DDOS attacks may sometimes manage to bypass the firewalls and do the damage to the server. Even though firewall is not a complete answer to online threats, it can most effectively handle the attacks and provide security to the computer up to the maximum possible extent.

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