Internet Security - Dang Thanh Binh's Page

22 INTERNET SECURITY
2.1.1.2 IP Addressing
Addresses belonging to three different layers of TCP/IP architecture are shown in Table 2.2
below.
• Physical (local or link) address: At the physical level, the hosts and routers are recognised
by their physical addresses. The physical address is the lowest-level address which
is specified as the node or local address defined by LAN or WAN. This local address
is included in the frame used by the network access layer. A local address is called a
physical address because it is usually (but not always) implemented in hardware. Ethernet
or token ring uses a six-byte address that is imprinted on the network interface card
(NIC) installed in the host or router. The physical address should be unique locally, but
not necessary universally. Physical addresses can be either unicast (one single recipient),
multicast (a group of recipients), or broadcast (all recipients on the network). The
physical addresses will be changed as a packet moves from network to network.
• IP address: An IP address is called a logical address at the network level because it
is usually implemented in software. A logical address identifies a host or router at the
network level. TCP/IP calls this logical address an IP address. Internet addresses can be
either unicast, multicast or broadcast. IP addresses are essentially needed for universal
communication services that are independent of underlying physical networks. IP
addresses are designed for a universal addressing system in which each host can
be identified uniquely. An Internet address is currently a 32-bit address which can
uniquely define a host connected to the Internet.
• Port address: The data sequences need the IP address and the physical address to
move data from a source to the destination host. In fact, delivery of a packet to a
host or router requires two levels of addresses, logical and physical. Computers are
devices that can run multiple processes at the same time. For example, computer A
communicates with computer B using TELNET. At the same time, computer A can
communicate with computer C using File Transfer Protocol (FTP). If these processes
occur simultaneously, we need a method to label different processes. In TCP/IP architecture,
the label assigned to a process is called a port address. A port address in
TCP/IP is 16 bits long.
The Internet Assigned Numbers Authority (IANA) manages the well-known port
numbers between 1 and 1023 for TCP/IP services. Ports between 256 and 1023 were
normally used by UNIX systems for UNIX-specific services, but are probably not
found on other operating systems.
Table 2.2 TCP/IP architecture and corresponding addresses
Layer TCP/IP Protocol Address
Application HTTP, FTP, SMTP
DNS and other protocols
Port address
Transport TCP, UDP —
Internet IP, ICMP, IGMP IP address
Network access Physical network Physical (link) address