The Internet

Chapter 13The InternetKnow how the Internet began.Understand the architecture of today’s Internet and its relation-ship with ISPs. Understand the importance of the TCP/IP protocol suite. After reading this chapter, the reader should be able to:OBJECTIVESUnderstand the role of IP, UDP, and TCP in the Internet. Understand the difference between the Internet, an intranet,and an extranet.HISTORYANDADMINISTRATION13.1RFCs can be found at 13-1Internet todayTechnical Focus: Maturity Levels of an RFCAn RFC, during its lifetime, falls into one of six maturity levels: proposed standard, draft standard, Internet standard, historic, experimental, and informational.Figure 13-2Internet administrationTCP/IPPROTOCOLSUITE13.2Figure 13-3TCP/IP protocol suiteFigure 13-4IP datagramTechnical Focus: Inside the Header of an IP DatagramAn IP datagram contains several fields. The most important are the source and destination addresses of the datagram (IP addresses). The header also contains fields related to fragmentation. The size of a datagram may be too large for some LAN or WAN protocols. In this case, the datagram is divided into fragments; each fragment carries the same identification number as well as other information to help the receiver assemble the datagram. The header also has two length fields; one defines the length of the header, the other defines the length of the entire packet. One field that can decrease traffic on the Internet holds the number of routers a packet can visit before it is discarded. The header also contains a checksum field to determine the validity of the packet.Figure 13-5Internet addressTechnical Focus: Classful versus Classless AddressingWith more and more organizations wanting to use the Internet, the Internet authorities are running out of IP addresses. Internet addresses were originally designed as classful addresses. By this, we mean that the total number of 32-bit addresses was divided unevenly into five classes: A, B, C, D, and E. Class A and B contain blocks of addresses with a very large range. Each block is granted to one organization, but most of these organizations never use their allotted number of addresses. This is a tremendous waste of addresses. Recently, a new design called classless addressing has been implemented. In this design, all available addresses are put into a big pool; each organization is granted a range of addresses according to its need. Figure 13-6A part of the InternetFigure 13-7UDP user datagramTechnical Focus: Inside a UDP headerThe header of the UDP datagram is very simple: it contains only four fields. One field defines the application program that has sent the packet (the source), and another defines the application program that is to receive the packet (the destination). Another field defines the length of the entire packet. The last field carries a checksum for error detection.Figure 13-8TCP segment formatTechnical Focus: Inside a TCP Segment HeaderThe header of a segment is very complicated and contains optional as well as mandatory fields. We briefly discuss just the required fields. One pair of fields defines the source and destination application programs. Another pair is used for error and flow control; one holds the unique sequence number, and the other holds the acknowledgment number. One field defines the size of the sliding window in the transport layer. The sliding window in the transport layer uses the same concept as the one in the data link layer (see Chapter 5). There are also flags that define the purpose of the segment (for connection establishment, for termination, for acknowledgment, and so on). The last required field carries a checksum for error detection.NEXTGENERATION13.3ACCESS TO THEINTERNET13.3PRIVATE NETWORKS:INTRANET AND EXTRANET13.5Technical Focus: Network Address Translation (NAT)A technology that is related to private networks is network address translation (NAT). The technology allows a site to use a set of private addresses for internal communication and a set of (at least one) global Internet addresses for communication with other sites.