How computer network is divided into levels

Application layer:

To accomplish specific network applications through the interaction between the application process

（DNS,DHCP,HTTP,SMTP,TELNET,FTP,P2P）

Transport Layer

Interprocess communication between hosts

1. The application process which data packets from the network to the (port number)
2. Reliable Transfer Service and unreliable transport services

Network layer

How to transfer data packets from the source network to the destination network

1. Identifying each network (network number part of the IP address)
2. Routing data packets (router routing protocols and forwarding protocols)

data link layer

How to transfer data packets from the source network to the destination network

1. Identification of the network interfaces of hosts and routers (MAC address)
2. Broadcast channel and the channel point
3. And reliable transmission error detection

Physical Layer

How hosts, routers and other network devices access the network

1. What identification signal using the bit 0 and 1 (signal modulation and coding)
2. What physical interfaces (mechanical size, number and arrangement of pins)

Computer Network Architecture hierarchical thinking, for example