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2012-05-31

A+: More on TCP/IP Configuration

TCP/IP Alternate Configuration


The Alternate Configuration tab is used to set up a different configuration for use when a DHCP server is not available or when a different set of user-configured settings are needed, as when a laptop is being used at a secondary location. By default, automatic private IP addressing (APIPA) is used when no DHCP server is in use. APIPA assigns each system a unique IP address in the 169.254.x.x range. APIPA enables a network to perform LAN connections when the DHCP server is not available, but systems using APIPA cannot connect to the Internet.  When you see a 169.254 series address, DHCP has failed, and should be repaired.

You can also use the Alternate Configuration tab to specify the IP address, subnet mask, default gateway, DNS servers, and WINS servers. This option is useful if this system is moved to another network that uses different IP addresses for these servers.


TCP/IP User-Configured IP and DNS Addresses

When a DHCP server is not used, the General tab is used to set up the IP address, subnet mask, default gateway, and DNS servers used by the network client .

TCP/IP User-Configured Advanced Settings

Click the Advanced button to bring up a multitabbed dialog for adding or editing gateways (IP Settings), DNS server addresses (DNS), adjusting WINS resolution (WINS), and adjusting TCP/IP port filtering (Options). These options can be used whether DHCP addressing is enabled or not.

Understanding IP Addressing, Subnet Masks, and IP Classes

An IPv4 address consists of a group of four numbers that each range from 0 to 255, for example: 192.168.1.1. IP addresses are divided into two sections: the network portion, which is the number of the network the computer is on, and the host portion, which is the individual number of the computer. Using the IP address we just mentioned as an example, the 192.168.1 portion would typically be the network number, and .1 would be the host number. A subnet mask is used to distinguish between the network portion of the IP address, and the host portion. For example, a typical subnet mask for the IP address we just used would be 255.255.255.0. The 255s correspond to the network portion of the IP address.

The subnet mask is also used to define subnetworks, if subnetworking is being implemented. Subnetworking goes beyond the scope of the A+ exam.

Both computers and other networked devices, such as routers and network printers, can have IP addresses, and some devices can have more than one IP address. For example, a router will typically have two IP addresses—one to connect the router to a LAN, and the other that connects it to the Internet, enabling it to route traffic from the LAN to the Internet and back.

IP addresses are divided into three major categories: Class A, Class B, and Class C, which define ranges of IP addresses. Class A is designated for large corporations, ISPs, and government. Class B is designated for mid-sized corporations and ISPs. Class C is designated for small offices and home offices.

Class Mask (default) IP range
A 255.0.0.0 0.0.0.0 - 127.255.255.255
B 255.255.0.0 128.0.0.0 - 191.255.255.255
C 255.255.255.0 192.0.0.0 - 223.255.255.255

The 127 network is reserved for testing. This is known as the loopback, for example 127.0.0.1. The usable starting IP for Class A is actually 1.0.0.0.

In any given network the first and last addresses are reserved and cannot be assigned to computers or other hosts. For example, in the 192.168.1.0 network, 192.168.1.1 through 192.168.1.254 can be assigned, but 192.168.1.0 is reserved for the network number, and 192.168.1.255 is reserved for something called the broadcast.

Each number in an IP address is called an octet. An octet is an 8-bit byte. This means that in the binary numbering system the number can range from 00000000—11111111. For example, 255 is actually 11111111 when converted to the binary numbering system. Another example: 192 equals 11000000.

To convert numbers from decimal to binary and vice-versa use the Windows calculator. Press Windows+R to bring up the Run prompt, then type calc. This will run the Windows Calculator. From here click View on the menu bar and select Scientific. Now you will notice radio buttons on the upper left that allow you to change between numbering systems. Simply type any number, and then select the numbering system you want to convert it to.

In a Class A network, the first octet is the network portion of the IP address, and the three remaining octets identify the host portion of the IP address. Class B networks use the first and second octets as the network portion, and the third and fourth octets as the host portion. Class C networks use the first three octets as network portion and the last octet as the host portion of the IP address.

The size of the network portion increases in octets, and the host portion decreases as you ascend through the classes. As time goes on, you will see more patterns like this within TCP/IP.

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