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Title: Internet Protocols
Description: It contains well detailed summaries on IP adressing and subnetting with well detailed examples. it contains well explained IP subnetting.
Description: It contains well detailed summaries on IP adressing and subnetting with well detailed examples. it contains well explained IP subnetting.
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C H A P TER
30
Internet Protocols
Background
The Internet protocols are the world’s most popular open-system (nonproprietary) protocol suite
because they can be used to communicate across any set of interconnected networks and are equally
well suited for LAN and WAN communications
...
The Internet protocol suite not only includes lower-layer
protocols (such as TCP and IP), but it also specifies common applications such as electronic mail,
terminal emulation, and file transfer
...
Discussions include IP addressing and key upper-layer protocols
used in the Internet
...
Internet protocols were first developed in the mid-1970s, when the Defense Advanced Research
Projects Agency (DARPA) became interested in establishing a packet-switched network that would
facilitate communication between dissimilar computer systems at research institutions
...
The result of this development effort was the Internet protocol
suite, completed in the late 1970s
...
Documentation of the Internet protocols (including new or revised protocols) and policies are
specified in technical reports called Request For Comments (RFCs), which are published and then
reviewed and analyzed by the Internet community
...
To illustrate the scope of the Internet protocols, Figure 30-1 maps many of the protocols of
the Internet protocol suite and their corresponding OSI layers
...
Internet Protocols 30-1
Internet Protocol (IP)
Figure 30-1
Internet protocols span the complete range of OSI model layers
...
IP is documented in RFC 791 and
is the primary network-layer protocol in the Internet protocol suite
...
IP has two primary
responsibilities: providing connectionless, best-effort delivery of datagrams through an
internetwork; and providing fragmentation and reassembly of datagrams to support data links with
different maximum-transmission unit (MTU) sizes
...
30-2
Internetworking Technology Overview, June 1999
IP Packet Format
Figure 30-2
Fourteen fields comprise an IP packet
...
•
Total Length—Specifies the length, in bytes, of the entire IP packet, including the data and
header
...
This field is used to help
piece together datagram fragments
...
The low-order bit specifies whether the packet can be fragmented
...
The third or
high-order bit is not used
...
•
Time-to-Live—Maintains a counter that gradually decrements down to zero, at which point the
datagram is discarded
...
•
Protocol—Indicates which upper-layer protocol receives incoming packets after IP processing is
complete
...
IP Header Length (IHL)—Indicates the datagram header length in 32-bit words
...
Source Address—Specifies the sending node
...
Internet Protocols 30-3
Internet Protocol (IP)
•
•
Options—Allows IP to support various options, such as security
...
IP Addressing
As with any other network-layer protocol, the IP addressing scheme is integral to the process of
routing IP datagrams through an internetwork
...
These IP addresses can be subdivided and used to create addresses for subnetworks,
as discussed in more detail later in this chapter
...
The network number identifies a network and
must be assigned by the Internet Network Information Center (InterNIC) if the network is to be part
of the Internet
...
The host number identifies a host on a
network and is assigned by the local network administrator
...
Each bit in the octet has a binary weight (128, 64, 32,
16, 8, 4, 2, 1)
...
Figure 30-3 illustrates the basic format of an IP address
...
32 Bits
Network
Host
8 Bits
8 Bits
8 Bits
8 Bits
Dotted
Decimal
Notation
172
•
16
•
122
•
204
IP Address Classes
IP addressing supports five different address classes: A, B,C, D, and E
...
The left-most (high-order) bits indicate the network class
...
30-4
Internetworking Technology Overview, June 1999
IP Address Classes
Table 30-1
Reference Information About the Five IP Address Classes
IP
Addre
ss
Class
Format
Purpose
A
N
...
H
...
Bits
Network/Host
Few large
organizations
0
1
...
0
...
0
...
0
7/24
16,777, 2142
(224 – 2)
N
...
H
...
1
...
0 to
191
...
0
...
N
...
H
Relatively small
organizations
1, 1, 0
192
...
1
...
255
...
0
22/8
245 (28 – 2)
D
N/A
Multicast groups
(RFC 1112)
1, 1, 1, 0
224
...
0
...
255
...
255
N/A (not for
commercial use)
N/A
E
N/A
Experimental
1, 1, 1, 1
240
...
0
...
255
...
255
N/A
N/A
1
2
Max
...
One address is reserved for the broadcast address, and one address is reserved for the network
...
(Note the high-order bits in
each class
...
Bits
Class A
0
IP address formats A, B, and C are available for commercial use
...
In an IP address of 172
...
1
...
Because 172 falls between 128 and 191, 172
...
1
...
Figure 30-5 summarizes the range of possible values for the first octet of each address class
...
First Octet
in Decimal
High-Order
Bits
Class A
1 Ð 126
0
Class B
128 Ð 191
10
Class C
192 Ð 223
110
Class D
224 Ð 239
1110
Class E
240 Ð 254
1111
24144
Address
Class
IP Subnet Addressing
IP networks can be divided into smaller networks called subnetworks (or subnets)
...
Subnets are under local administration
...
A given network address can be broken up into many subnetworks
...
16
...
0,
172
...
2
...
16
...
0, and 172
...
4
...
16
...
0
...
)
IP Subnet Mask
A subnet address is created by “borrowing” bits from the host field and designating them as the
subnet field
...
Figure 30-6
shows how bits are borrowed from the host address field to create the subnet address field
...
Class B Address: Before Subnetting
1
0
Network
Network
Host
Host
1
0
Network
Network
Subnet
Host
Class B Address: After Subnetting
Subnet masks use the same format and representation technique as IP addresses
...
Figure 30-7 illustrates a sample subnet mask
...
Network
Subnet
Host
Binary
representation
11111111
11111111
11111111
00000000
Dotted decimal
representation
255
255
255
0
24145
Network
Subnet mask bits should come from the high-order (left-most) bits of the host field, as Figure 30-8
illustrates
...
Class A addresses are not discussed
in this chapter because they generally are subnetted on an 8-bit boundary
...
64
32
16
8
4
2
1
1
0
0
0
0
0
0
0
=
128
1
1
0
0
0
0
0
0
=
192
1
1
1
0
0
0
0
0
=
224
1
1
1
1
0
0
0
0
=
240
1
1
1
1
1
0
0
0
=
248
1
1
1
1
1
1
0
0
=
252
1
1
1
1
1
1
1
0
=
254
1
1
1
1
1
1
1
1
=
255
24146
128
Various types of subnet masks exist for Class B and C subnets
...
255
...
0, while the subnet
mask for a Class B address 171
...
0
...
255
...
0
...
The subnet mask for a Class C address 192
...
2
...
255
...
248
...
The reference charts shown in table 30–2 and table 30–3 can be used when planning Class B and C
networks to determine the required number of subnets and hosts, and the appropriate subnet mask
...
255
...
0
2
16382
3
255
...
224
...
255
...
0
14
4094
5
255
...
248
...
255
...
0
62
1022
7
255
...
254
...
255
...
0
254
254
9
255
...
255
...
255
...
192
1022
62
11
255
...
255
...
255
...
240
4094
14
Internetworking Technology Overview, June 1999
IP Address Classes
Number of Bits
Subnet Mask
Number of Subnets
Number of Hosts
13
255
...
255
...
255
...
252
16382
2
Table 30-3
Class C Subnetting Reference Chart
Number of Bits
Subnet Mask
Number of Subnets
Number of Hosts
2
255
...
255
...
255
...
224
6
30
4
255
...
255
...
255
...
248
30
6
6
255
...
255
...
First, the router extracts the IP destination address from the incoming packet and retrieves
the internal subnet mask
...
This causes the host portion of the IP destination address to be removed, while the destination
network number remains
...
Finally, it forwards the frame to the destination IP address
...
Logical AND Operation
Three basic rules govern logically “ANDing” two binary numbers
...
Second, 1 “ANDed” with 0 yields 0
...
The truth table provided
in table 30–4 illustrates the rules for logical AND operations
...
Figure 30-9 illustrates that when a logical AND of the destination IP address and the subnet mask is
performed, the subnetwork number remains, which the router uses to forward the packet
...
Network
Destination IP
Address
Subnet
Host
10101011
00010000
00000001
00000010
255
...
255
...
16
...
2
00010000
00000001
00000000
16
1
171
0
Address Resolution Protocol (ARP) Overview
For two machines on a given network to communicate, they must know the other machine’s physical
(or MAC) addresses
...
After receiving a MAC-layer address, IP devices create an ARP cache to store the recently acquired
IP-to-MAC address mapping, thus avoiding having to broadcast ARPS when they want to recontact
a device
...
In addition to the Reverse Address Resolution Protocol (RARP) is used to map MAC-layer addresses
to IP addresses
...
RARP relies on the presence of a RARP server
with table entries of MAC-layer-to-IP address mappings
...
In today’s terminology, however,
the term gateway refers specifically to a device that performs application-layer protocol translation
between devices
...
These are known as autonomous systems
...
Routers within the Internet are organized hierarchically
...
The Routing Information Protocol (RIP) is an example
of an IGP
...
These
routers use an exterior gateway protocol to exchange information between autonomous systems
...
Note Specific routing protocols, including BGP and RIP, are addressed in individual chapters
presented in Part 6 later in this book
...
Dynamic routing calls for routes to be calculated automatically at
regular intervals by software in routing devices
...
An IP routing table, which consists of destination address/next hop pairs, is used to enable dynamic
routing
...
31
...
0, send the packet out Ethernet interface 0 (E0)
...
The entire route
is not known at the onset of the journey, however
...
Each node’s involvement in the routing process is limited to forwarding packets based on internal
information
...
This task is left to
another Internet protocol, the Internet Control-Message Protocol (ICMP), which is discussed in the
following section
...
ICMP is documented in RFC 792
...
If an ICMP
message cannot be delivered, no second one is generated
...
When an ICMP destination-unreachable message is sent by a router, it means that the router is unable
to send the package to its final destination
...
Two reasons
exist for why a destination might be unreachable
...
Less frequently, the router does not have a route to the destination
...
Network-unreachable messages usually mean that a
failure has occurred in the routing or addressing of a packet
...
Protocol-unreachable messages generally
mean that the destination does not support the upper-layer protocol specified in the packet
...
An ICMP echo-request message, which is generated by the ping command, is sent by any host to test
node reachability across an internetwork
...
An ICMP Redirect message is sent by the router to the source host to stimulate more efficient
routing
...
ICMP redirects allow host
routing tables to remain small because it is necessary to know the address of only one router, even if
that router does not provide the best path
...
Internet Protocols 30-11
Transmission Control Protocol (TCP)
An ICMP Time-exceeded message is sent by the router if an IP packet’s Time-to-Live field
(expressed in hops or seconds) reaches zero
...
The router then
discards the original packet
...
Each router periodically multicasts Router-Advertisement
messages from each of its interfaces
...
Hosts can use Router-Solicitation messages to request
immediate advertisements rather than waiting for unsolicited messages
...
Primarily, it does not require hosts to recognize routing protocols, nor does it require manual
configuration by an administrator
...
If a host uses a poor first-hop router to reach
a particular destination, it receives a Redirect message identifying a better choice
...
TCP corresponds to the
transport layer (Layer 4) of the OSI reference model
...
With stream data transfer, TCP delivers an unstructured stream of bytes identified by sequence
numbers
...
Instead, TCP groups bytes into segments and passes them to IP for delivery
...
It does this by sequencing bytes with a forwarding acknowledgment number that
indicates to the destination the next byte the source expects to receive
...
The reliability mechanism of TCP allows devices
to deal with lost, delayed, duplicate, or misread packets
...
TCP offers efficient flow control, which means that, when sending acknowledgments back to the
source, the receiving TCP process indicates the highest sequence number it can receive without
overflowing its internal buffers
...
Finally, TCP’s multiplexing means that numerous simultaneous upper-layer conversations can be
multiplexed over a single connection
...
Connection establishment is performed by using a “three-way handshake” mechanism
...
This mechanism also guarantees that both sides are ready to transmit data
and know that the other side is ready to transmit as well
...
30-12
Internetworking Technology Overview, June 1999
Positive Acknowledgment and Retransmission (PAR)
Each host randomly chooses a sequence number used to track bytes within the stream it is sending
and receiving
...
The second host (Host B) receives the SYN,
records the sequence number X, and replies by acknowledging the SYN (with an ACK = X + 1)
...
An ACK = 20 means the host has
received bytes 0 through 19 and expects byte 20 next
...
Host A then acknowledges all bytes Host B sent with a forward acknowledgment
indicating the next byte Host A expects to receive (ACK = Y + 1)
...
Positive Acknowledgment and Retransmission (PAR)
A simple transport protocol might implement a reliability-and-flow-control technique where the
source sends one packet, starts a timer, and waits for an acknowledgment before sending a new
packet
...
Such a technique is called positive acknowledgment and retransmission (PAR)
...
The sequence numbers are sent
back in the acknowledgments so that the acknowledgments can be tracked
...
TCP Sliding Window
A TCP sliding window provides more efficient use of network bandwidth than PAR because it
enables hosts to send multiple bytes or packets before waiting for an acknowledgment
...
Because TCP provides a
byte-stream connection, window sizes are expressed in bytes
...
Initial
window sizes are indicated at connection setup, but might vary throughout the data transfer to
provide flow control
...
”
In a TCP sliding-window operation, for example, the sender might have a sequence of bytes to send
(numbered 1 to 10) to a receiver who has a window size of five
...
It would then wait for an
acknowledgment
...
In the same packet, the receiver would indicate that its window size is 5
...
The
receiver would respond with an ACK = 11, indicating that it is expecting sequenced byte 11 next
...
At this point, the sender cannot send any more bytes until the receiver sends another
packet with a window size greater than 0
...
Figure 30-10
Twelve fields comprise a TCP packet
...
In the connection-establishment phase, this field also can be used to identify an initial
sequence number to be used in an upcoming transmission
...
•
•
•
Data Offset—Indicates the number of 32-bit words in the TCP header
...
•
•
•
•
30-14
Source Port and Destination Port—Identifies points at which upper-layer source and destination
processes receive TCP services
...
Reserved—Remains reserved for future use
...
Urgent Pointer—Points to the first urgent data byte in the packet
...
Data—Contains upper-layer information
...
UDP is basically an interface between IP and upper-layer
processes
...
Unlike the TCP, UDP adds no reliability, flow-control, or error-recovery functions to IP
...
UDP is useful in situations where the reliability mechanisms of TCP are not necessary, such as in
cases where a higher-layer protocol might provide error and flow control
...
The UDP packet format contains four fields, as shown in Figure 30-11
...
Figure 30-11
A UDP packet consists of four fields
...
A length field specifies the length of the UDP
header and data
...
Internet Protocols Application-Layer Protocols
The Internet protocol suite includes many application-layer protocols that represent a wide variety
of applications, including the following:
•
•
File Transfer Protocol (FTP)—Moves files between devices
•
•
Telnet—Serves as a terminal emulation protocol
•
Network File System (NFS), External Data Representation (XDR), and Remote Procedure Call
(RPC)—Work together to enable transparent access to remote network resources
•
•
Simple Mail Transfer Protocol (SMTP)—Provides electronic mail services
Simple Network-Management Protocol (SNMP)—Primarily reports anomalous network
conditions and sets network threshold values
X Windows—Serves as a distributed windowing and graphics system used for communication
between X terminals and UNIX workstations
Domain Name System (DNS)—Translates the names of network nodes into network addresses
Table 30-5 lists these higher-layer protocols and the applications that they support
Title: Internet Protocols
Description: It contains well detailed summaries on IP adressing and subnetting with well detailed examples. it contains well explained IP subnetting.
Description: It contains well detailed summaries on IP adressing and subnetting with well detailed examples. it contains well explained IP subnetting.