part 4-network.pdf - U2577 Computer Networks Part 4 Network...

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U2577 Computer Networks Part 4: Network Layer The Internet is a network of networks , illustrated in Fig. 1. We have known that switches use the spanning tree protocol to seek a unique path in a LAN, e.g., the small networks in the figure. In this part, we will explore how to send data through routers in the Internet, e.g., the whole network in the figure. A router is basically a super switch , which not only implements the link layer functions, but implements the network layer functions to deal with the huge Internet. R1 R2 A B C D E F G H J K L A B C D E F G R1 R2 H J K L Figure 1: Interconnected networks through routers. 1 Addressing The location-independent MAC addresses are not efficient in the huge Internet; instead, routers use another addressing mechanism, called the IP addresses , while the forwarding table in a router maps destination IP addresses to ports. Thus, in addition to a MAC address, each interface (i.e., a network adapter) of a node in the Internet has another unique name specified by a 32-bit string, called the IP address (Internet Protocol Address), e.g., 120.126.141.162. Note that 255.255.255.255 is the broadcast IP address. In general, the IP addresses are based on the locations of the nodes, as shown in Fig. 2. The idea of the location-based IP addressing is similar to the home addressing to facilitate a global mail-route selection. part 4 1/19
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1 2 4 3 N 5 1.1 1.2 1.N N.N N.2 N.1 Figure 2: IP addressing. In general, there are three types of the IP addresses: (1) permanent IP addresses; (2) dynamic IP addresses; (3) private IP addresses. 1.1 Permanent IP addresses In order to obtain a block of IP addresses for use within an organization, a network admin- istrator might first contact its Internet service provider (ISP), which would provide addresses from a larger block of addresses that had already been allocated to the ISP. Is there a global authority that has ultimate responsibility for managing the IP address space and allocating address blocks to ISPs and other organizations? Indeed there is! IP addresses are managed under the authority of the Internet Corporation for Assigned Names and Numbers (ICANN). Once an organization has obtained a block of addresses, it can assign permanent individual IP addresses to nodes in its organization. 1.2 Dynamic IP addresses In the late 1990s, Internet engineers realized that one would run out of IP addresses in a few years. They then developed a new addressing scheme that uses 128 bits (i.e., IPv6) instead of 32 (i.e., IPv4). However, these new addresses require a new version of the Internet Protocol, which necessitates some considerable amount of configuration work in thousands of routers. The dynamic host configuration protocol (DHCP) is one mechanism that reduces the number of IPv4 addresses needed by allocated them temporarily and dynamically instead of permanently.
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