IPv4 addresses have a structure that identifies makes the address unique as well as identifying the network the address belongs to.
The logical 32-bit IPv4 address is hierarchical and is made up of two parts, the network and the host. In the figure, the network portion is blue, and the host portion is red. Both parts are required in an IPv4 address. Both networks have the subnet mask 255.255.255.0. The subnet mask is used to identify the network on which the host is connected.
For example, in Figure 8-1 there is a host with an IPv4 address 192.168.5.11 with a subnet mask of 255.255.255.0. The first three octets, (192.168.5), identify the network portion of the address, and the last octet, (11) identifies the host. This is known as hierarchical addressing because the network portion indicates the network on which each unique host address is located. Routers only need to know how to reach each network, rather than needing to know the location of each individual host.
Figure 8-1 Example of Network Number and Host Number
With IPv4 addressing, multiple logical networks can exist on one physical network if the network portion of the logical network host addresses is different. For example: three hosts on a single, physical local network have the same network portion of their IPv4 address (192.168.18) and three other hosts have different network portions of their IPv4 addresses (192.168.5). The hosts with the same network number in their IPv4 addresses will be able to communicate with each other, but will not be able to communicate with the other hosts without the use of routing. In this example, there is one physical network and two logical IPv4 networks.
Another example of a hierarchical network is the telephone system. With a telephone number, the country code, area code, and exchange represent the network address and the remaining digits represent a local phone number.
The following is a summary of each topic in the chapter and some questions for your reflection.
What Did I Learn in this Module? (8.3.1)
• Purpose of the IPv4 Address— The IPv4 address is a logical network address that identifies a particular host. It must be properly configured and unique within the LAN, for local communication. It must also be properly configured and unique in the world, for remote communication.
An IPv4 address is assigned to the network interface connection for a host. This connection is usually a NIC installed in the device.
Every packet sent across the internet has a source and destination IPv4 address. This information is required by networking devices to ensure the information gets to the destination and any replies are returned to the source.
• The IPv4 Address Structure—The logical 32-bit IPv4 address is hierarchical and is made up of two parts, the network, and the host. As an example, there is a host with an IPv4 address 192.168.5.11 with a subnet mask of 255.255.255.0. The first three octets, (192.168.5), identify the network portion of the address, and the last octet, (11) identifies the host. This is known as hierarchical addressing because the network portion indicates the network on which each unique host address is located.
Routers only need to know how to reach each network, rather than needing to know the location of each individual host. With IPv4 addressing, multiple logical networks can exist on one physical network if the network portion of the logical network host addresses is different.
It makes sense that every device on the network has an IP address, and routers use these addresses to send packets from the source to the destination. When I send a letter through the mail, I put my address and the address of the recipient on the envelope. But now I see the other connection to how networks operate. The postal code and city of my recipient is a little bit like the network portion of the IP address, and the street address is like the host portion of the IP address. Can you think of any other analogies to network operations and IP addresses?