TCP/IP: Transmission Control Protocol / Internet Protocol and Network Address Translation (NAT) explained:

Two people can communicate effectively when they agree to use a common language. Computers work the same way. TCP/IP is the language computers use to speak with each other on the Internet. More specifically, TCP/IP is a set of rules or protocols  that defines how computers address each other and send data to each other.

Every computer on a TCP/IP network needs to have a unique address, similar to you needing a unique address for your house. TCP/IP addresses are based on 32-bit addresses. But rather than working with 32 1s and 0s in the Binary number system, we use decimals to represent IP addresses. Specifically, we use four decimal numbers separated by periods. These four decimal numbers represent the 32 binary digits separated into four equal parts called octets.  An octet is 8 bits. The highest decimal number that an octet can contain is 255. The decimal number 256 cannot be represented with 8 bits; therefore, no IP address has a number greater than 255 in any octet. A typical IP address: which just happens to be

When you connect to the Internet directly, using a dial up modem or broadband connection, your ISP typically assigns you an IP address from a pool of addresses that it owns.  These addresses are PUBLIC; their location is listed in routing tables that are a crucial part of the Internet's standard procedure to guide packets of data as they move from point to point. When you click a link to a Web page or check for new messages on your email server, the outgoing packet includes your IP address, the server on the other end of the connection returns the requested data to your IP address, and the Internet sees to it that those packets are routed to your computer properly.

On a Home or Small Office Network, having a unique Public IP address for every computer is unnecessary and possible dangerous. By sharing an Internet Connection instead, you can get by with a single Public IP address assigned to a single hardware device (a ROUTER).  Each of the computers on the local network has a private IP address that is NOT REACHABLE from the outside world but is known to other computers on the local network.  To communicate with Web Sites, external Email servers, and other Internet hosts, computers on the network funnel their requests through the Router on the edge of the Network, the one with the Public IP Address.  As each packet goes out onto the Internet, the gateway device makes a note of where it came from. When the return packets arrive, the gateway machine uses NAT to pass those packets back to the correct Private IP address on the network.

The Internet Assigned Numbers Authority (IANA) has reserved three blocks of the IP address space for use on private networks that are not directly connected to the Internet:

starting with to  to to


Routers, switches and residential gateways that use NAT almost always assign addresses from these private ranges.  The Internet Connection Sharing feature in Windows XP and Vista, assigns private IP addresses in the 192.168.0.x range. Unlike public IP addresses, which must be unique across the entire Internet, private IP addresses need be unique only on your local network. Although it is technically possible to use IP addresses from publicly available ranges on a private network, doing so is not recommended.

Using private IP addresses offers a significant security advantage because the computer or router that is managing the connection through NAT can inspect each incoming packet and decide whether to forward it or drop it. If a computer on the local network requested the connection, the NAT gateway will forward it; on the other hand, if a computer outside the network is trying to make an unsolicited connection, the gateway assumes that the traffic is hostile and discards it.

When you check your IP Address using IPCONFIG /ALL and it appears in the 169.254.x.y range and you can't access the Internet, your router is not assigning the private address.  This range of addresses is assigned by your computer using a feature called Automatic Private IP Addressing (APIPA). APIPA kicks in only when no DHCP server is available. If you're using ICS or a router or residential gateway that automatically assigns IP addresses and you computer is unable to acquire an IP address from the DHCP server, APIPA assigns a default address. This problem is often caused by a faulty network connection or by a firewall that is configured incorrectly. Disable any third-party security software, confirm that all physical connections are set up properly, and run through the Network Setup Wizard to see if you can repair the problem.


To view your private IP Address and Gateway address, click on START then on RUN, then type CMD and click on OK.



When the MS-DOS prompt window opens, type IPCONFIG /ALL and press return.


Looking at the example above: Line three is where ipconfig /all was typed and the lines below that are the result. The private address of this computer is on the line IPV4 which is

The router has been set to assign addresses in this range. The address of the router is Use this IP address in the address field of Internet Explorer to gain access to the routers configuration files.

Using the PING command.  Now that you know the address assigned to your NIC and of your router you can use the PING command to test the connection.

Still in the DOS Command Prompt Window, type PING the IP address of your router and you should receive the following:


Notice the Ping command on line three then the results. The Ping command sends out four pings and waits for a Reply. Look at Ping Statistics;   0 percent loss.   This shows that there is a good connection between the PC and the Router.

You can also ping a URL as show below:


In this example, I Pinged This ping proves a good Internet connection.

The Internet Protocol that is standard today was created in the early 1970's. The current version is Internet Protocol Version 4 (IPv4).  Due to the enormous growth of the Internet, IPv4 is no longer viable. IPv6 will soon replace IPv4 as the Internet standard. IPv4 is running out of network addresses, and routing tables are overflowing.

In IPv4, there are 32 bits in every address. This means that there are 2^32 or 4,294,969,296 unique IP addresses, and we are running out.

IPv6 uses 128 bit addresses, or 2^128.  The number is too long to type on one line.

Remember IPv4 address looks like this:

This is what an IPv6 IP address will look like: EFDC:BA62:7654:3201:EFDC:BA72:7654:3210 where all 8 octets are shown and is known as the FULL address.

Some addresses will have several 0's within the address and to make notation of this address easier, the nonsignificant and leading 0's can be dropped using what is called Double-Colon Notation.

1080:0000:0000:0000:0008:0800:200C:417A becomes 1080::8:800:200C:417A.

IPv6 will take several years to implement because it requires modification of the entire infrastructure of the internet. This is where NAT comes to the rescue. Network Address Translation allows a single device, such as a router, to act as an agent between the internet, the public network, and a local network, the private network,. This means that only a single, unique IP address is required to represent an entire group of computers.

Implementing dynamic NAT automatically creates a firewall between your internal network and the internet.  NAT only allows connections that originate inside the domain. Essentially, this means that a computer on an external network cannot connect to your computer unless your computer has initiated the contact. You can browse the Internet and connect to a site, and download a file, but somebody else cannot latch onto your IP address and use it to connect to a port on your computer.


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