Consequently, every layer:
- Has limited, defined tasks
- Has a precisely defined interface to the neighboring higher and lower
layers.
- Attaches its own layer-specific header to the data package being
passed on. The corresponding layer on the other side interprets and removes
the header.
The individual layers of the OSI model are listed in Table1-1.
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Provides a service for managing the application. |
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Manages the presentation of the data to be
independent of the architecture. |
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Administers communication relationships. |
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Makes sure that messages reach their destination
system via an optimal transmission path. |
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Manages data addressing and delivery between networks,
as well as fragmenting data for the Data Link Layer. A router functions at this layer (routing). |
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Manages the delivery of data across the physical
network. This layer provides error detection and packet framing. A bridge/switch functions at this layer. |
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Describes the network hardware, including electrical
signal characteristics such as voltage and current. A repeater or hub function at this layer. |
The TCP/IP protocol suite is structured as a hierarchy of five layers, sometimes referred to collectively as a protocol stack. This architectural scheme provides the following benefits:
- Each layer is designed for a specific purpose and exists on both the
sending and receiving hosts.
- Each layer is designed so that a specific layer on one machine sends
or receives exactly the same object sent or received by its peer process
on another machine.
- Each layer on a host acts independently of other layers on the same
machine, and in concert with the same layer on other hosts.
Table1-2 lists each layer in the TCP/IP network model.
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Consists of user-accessed application programs and
network services. This layer is also responsible for defining the way in which cooperating networks represent data. A gateway functions at this layer. |
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Manages the transfer of data using acknowledged and
unacknowledged transport protocols. This layer also manages the connections between cooperating applications. |
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Manages data addressing and delivery between networks, as
well as fragmenting data for the network interface layer. A router functions at this layer. |
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Manages the delivery of data across the physical network.
This layer provides error detection and packet framing. A bridge/switch functions at this layer. |
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Describes the network hardware, including electrical signal
characteristics such as voltage and current. A repeater or hub functions at this layer. |
Computing Network Numbers
Given this definition of the logical AND operator, consider the
IPv4 address of 171.63.14.3, a Class B address whose netmask is, therefore,
255.255.0.0. Figure 1-1 illustrates how a network number is computed.
| IPv4 address in decimal | 171.63.14.3 |
| IPv4 address in binary | 10101011 00111111 00001110 00000011 |
| Class B netmask in decimal | 255.255.0.0 |
| Class B netmask in binary | 11111111 11111111 00000000 00000000 |
| Apply the logical AND operator | |
| IPv4 Address (decimal) | 171.63.14.3 |
| IPv4 Address (binary) | 10101011 00111111 00001110 00000011 |
| AND netmask | 11111111 11111111 00000000 00000000 |
| Network # (binary) | 10101011 00111111 00000000 00000000 |
| Network # (decimal) | 171.63.0.0 |
Thus, the resulting network number is 171.63.0.0 in decimal. Notice that the host portion of the address is zero (masked out).
Table1-3 shows the possible Class C subnet masks.
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Table1-4 shows the possible Class B subnet masks.
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Easy way to determine subnets, network numbers, broadcast numbers for a subnetted Class C network.
Given : 192.168.11.140/27
Convert IP address to binary : 11000000 10101000 00001011 10001100
A Class C network masks the first 24 bits (from the left). In this example, we are masking 27 bits. That means the 3 bits, on the left, of 10001100 will be used for the network and the remaining 5 bits will be used for hosts.
# of subnets available = 23 = 8 (the power is the number of additional bits beyond the 24 that will be used for the network)
# of hosts available = 25 - 2 = 30 (the power is the number of bits used for hosts) Note: we subtract the first and last because those are reserved for the network number and the broadcast number.
So, here are the network and broadcast numbers for each of the subnets:
(Our IP address is on the 100 subnet)
Subnets are all binary permutations, of the available bits.
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Modified by Marty Froomin on 4/7/04