How the Internet Works: A Dialup Example

Please refer to the diagram below. In this configuration, a dial-up user has established a switched, point-to-point modem circuit through the telephone network. This represents Layer 1 of the OSI model, the "Physical" layer.

The PC also runs a SLIP (Serial Link IP) or PPP (Point-Point Protocol) program which commmunicates to your ISP or business' Communication Server(s). This represents Layer 2 of the OSI model, the "Logical Link" layer.

The OSI Model

The Open Systems Interconnection (OSI) model defines 7 layers of data communications protocol. A protocol is what defines how computers and people talk to each other.

Layer 1: Physical Layer

The Physical Layer, Layer 1, interfaces the network devices with the network medium. It is usally defined by electrical and mechanical standards. For our example, Layer 1 is the dial-up telephone network.
Layer 2: Logical Link Layer

Layer 2, the Logical Link Layer, defines how framing and physical data flow occur in the network. This is sometimes called the "Frame Layer", from the old X.25 system era. It also encompasses the the Q.921 standard used in ISDN and Frame Relay applications. For our example. Layer 2 will consist of one or two protocols. These are known as SLIP (Serial Link Internet Protocol) and PPP (Point-to-Point Protocol). In the example diagram, these "Frames" are represented by Yellow rectangles. It should be pointed out that these SLIP or PPP frames do not traverse the Internet. They terminate at your ISP's Communications Server and the Client's (customer's) PC.
Layer 3: Network Layer

The Network Layer, Layer 3, is responsible for the addressing and delivery of "packets" of information. This is sometimes referred to as the "Packet Layer", from the old X.25 packet switching days. In our example, the Network Layer consists of IP (Internet Protocol) packets. These packets are generated by the PC's "Packet Driver" or "Dialer" program and do travel through the Internet. Each of these packets has a "Source" address (the Client's IP address) and a "Destination" address.
Since IP addresses are hard to remember, things are often represented by a mneumonic, alphabetical-based address, like telecom.tbi.net, for example. So what happens when you click on a hyperlink on the World Wide Web is that first, you query a local "Name Server". The "Name Server" converts the alphabetical address into a number, which is then sent back to the Client's PC. The Client's PC then outputs an IP packet to the destination address. In our example, these IP packets are indicated by Blue squares.
The Network Layer supports is own special sub-protocol known as the Internet Control Message Protocol (ICMP). This is used in a popular program known as PING (Packet InterNet Groper) which can be used to see if a particular IP address is alive or dead.
Layer 4: Transport Layer

The Transport Layer is responsible for managing Network Layer connections and providing for reliable packet delivery (or not). There is no X.25 equivalent name. In our example, the Internet will use two types of packets in the Transport Layer. These are known as UDP (User Datagram Protocol) and TCP (Transmission Control Protocol).
The Transport Layer adds "Port" addresses to packets for both the Source and Destination IP addresses. If someone runs a "standard" service, the Destination port is said to be a "Well-Known-Port". WWW servers usually run on Port 80. Telnet (Terminal Emulation/BBS) systems typically run on Port 23. FTP on port 21. Post Office Protocol (POP) on port 110.
When an Intelligent terminal (e.g. PC) accesses a port, it "plugs" into it. In the UNIX world, these TCP/IP ports are known as "sockets". In the Microsoft PC world, a special program known as a "Winsock" is used to create these network socket connections.
Layer 5: Session Layer

The Session Layer is not used for most standard Internet stuff. It does carry a protocol called NetBEUI (NetBIOS Extended User Interface) which is used in some popular Network Operating Systems by IBM and Microsoft. Even though NetBEUI doesn't really interact with Layer 4, the Transport Layer, it still generates UDP or TCP-based packets. Microsoft's NetBIOS is an unusual beast! It can interact with the TCP/IP driver, or talk to the LAN Card at layer 2 via NetBEUI! In fact, default Microsoft implementations will "bind" SMB/NetBIOS (Client for Microsoft Networks) to both the TCP/IP protocol and a LAN card (NetBEUI), if available, taking advantage of either/both resources!
NetBEUI is usually used as a transport mechanism for LAN-based systems, such as Ethernet or Token Ring and not in dialup systems. However, it is worthy of note, since it is a popular protocol for Microsoft Network Operating Systems.
Layer 6: Presentation Layer

The Presentation Layer is responsible for handling architecture-independent data formats, including data coding, encryption, and compression. Again, this layer is not used for most standard Internet services. It is used to transport a protocol called NetBIOS, which is a LAN-based Network Operating System protocol implemented by IBM and Microsoft. This protocol usually interacts with NetBEUI, at Layer 5 in a classic LAN configuration, but can also interact directly with the TCP/IP driver, if desired.

It is the latter configuration that allows easy dialup Network Operating System functions over TCP/IP. The NetBIOS driver communicates with the TCP/IP stack. Microsoft sometimes refers to this as NBT (NetBIOS transport over TCP/IP).
Layer 7: Application Layer

This layer provides an interface to the end-user. This is the real "meat and potatoes" of the Internet. In a Network Operating System (SAMBA), the Server Message Block (SMB) protocol will interact directly with NetBIOS at Layer 6. But in most Internet applications, protocols at this layer will interface directly with the Transport Layer (Layer 4), taking advantage of either the UDP or TCP services provided by Layer 4. In most cases, the TCP services will be used in your Internet excursions and many people refer to this collective set of services as TCP/IP.

Some TCP-based applications include FTP (File Transfer Protocol), HTTP (HyperText Transfer Protocol - WWW), TELNET (Terminal Emulation), SMTP (Simple Mail Transfer Protocol), POP (Post Office Protocol), UNIX remote printing (LPR), UNIX Remote Login (RLOGIN), UNIX Remote Execution (REXEC), UNIX X-Windows, and NetBIOS session data.

Some UDP-based applications include RADIUS (Remote Authentication Dial In User System), TFTP (Trivial File Transfer Protocol), SNMP (Simple Network Management Protocol), NTP (Network Time Protocol), BOOTP (Bootstrap Protocol), DHCP (Dynamic Host Configuration Protocol), and NetBIOS Datagram and Nameserver protocols.