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Unformatted text preview: addresses, humans work best using the name of a host. In the TCP/IP world the Domain Name
System (DNS) is a distributed database that provides the mapping between IP addresses and
hostnames. Chapter 14 looks into the DNS in detail.
For now we must be aware that any application can call a standard library function to look up
the IP address (or addresses) corresponding to a given hostname. Similarly a function is
provided to do the reverse lookup-given an IP address, look up the corresponding hostname.
Most applications that take a hostname as an argument also take an IP address. When we use the
Telnet client in Chapter 4, for example, one time we specify a host-name and another time we
specify an IP address. 1.6 Encapsulation
When an application sends data using TCP, the data is sent down the protocol stack, through
each layer, until it is sent as a stream of bits across the network. Each layer adds information to
the data by prepending headers (and sometimes adding trailer information) to the data that it
receives. Figure 1.7 shows this process. The unit of data that TCP sends to IP is called a TCP
segment. The unit of data that IP sends to the network interface is called an IP datagram. The
stream of bits that flows across the Ethernet is called a frame.
The numbers at the bottom of the headers and trailer of the Ethernet frame in Figure 1.7 are the
typical sizes of the headers in bytes. We'll have more to say about each of these headers in later
A physical property of an Ethernet frame is that the size of its data must be between 46 and 1500
bytes. We'll encounter this minimum in Section 4.5 and we cover the maximum in Section 2.8.
All the Internet standards and most books on TCP/IP use the term octet instead of byte. The use of this cute, but
baroque term is historical, since much of the early work on TCP/IP was done on systems such as the DEC-10,
which did not use 8-bit bytes. Since almost every current computer system uses 8-bit bytes, we'll use the term
byte in this text.
To be completely ac...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12