A server is a computer or device on a network that manages network resources. Servers are often dedicated and they perform only their server tasks.
D (Network and subnet parts) . When using classful IP addressing concepts as described in Chapter 13, Analyzing Subnet Masks, addresses have three parts: network, subnet, and host. For addresses in a single classful network, the network parts must be identical for the numbers to be in the same network.
Every email message consists of two parts, the body and the header. The header can be thought of as the envelope of the message, containing the address of the sender, the recipient, the subject and other information. The body contains the actual text and the attachments.