Giao thông vận tải thông điệp và phân chia nó thành các phần khác nhau, được gọi là các gói tin. Mỗi của các bộ phận này sau đó được gửi qua mạng bằng cách sử dụng một lược đồ địa chỉ được xác định bởi các giao thức lớp Internet. | 28 Part I Introducing Firewall Basics When you send an e-mail message to a mail server a mail protocol defines how the message should be formatted and what commands the mail server understands. This mail protocol operates at the Application layer. Before this mail message is sent out a Transport layer protocol takes the message and divides it into different parts called packets. Each of these parts is then sent across the network using an addressing scheme that is defined by Internet layer protocols. Network Interface layer protocols define how the packets are sent out across the physical network medium. Just to make things a little more complicated at this level the information is sent in chunks of data that are referred to as datagrams which are nothing more than packets by another name. At the receiving side protocols at each layer determine how the message is received how the address is confirmed how packets are reassembled into a mail message and how the mail server interprets the mail message. How is using layers to understand the nature of a network related to better understanding how firewalls work Most traditional firewalls focus on the Internet and Transport layers. These layers define where network packets come from for whom they re intended and whether a packet fits correctly into a sequence of related packets. More advanced firewalls however also operate at the Application layer. Inspecting traffic at the Application layer means that a firewall understands how packets combine to form a larger data exchange such as an entire e-mail message and the structure of that e-mail message. Before we explore this further we want to cover in a bit more depth how computers send network packets to each other. The Numbers Game Address Basics Just as you need telephone numbers and addresses to send messages to your friends computers need addresses to reliably communicate with each other. Take a look at some of the addressing schemes and how they are used Hardware .
