Information Technology Reference
In-Depth Information
support such large MTUs. For example, the MTU for Ethernet is approximately 1518 bytes.
When the IP layer receives a packet to send, it first queries the outgoing interface to get its
MTU. If the packet's size is greater than the interface's MTU, the layer fragments the packet.
When a packet is fragmented, it is not reassembled until it reaches the destination IP layer.
The destination IP layer performs the reassembly. Any router in the path can fragment a
packet, and any router in the path can fragment a fragmented packet again. Each frag-
mented packet receives its own IP header and is routed independently from other packets.
Routers and Layer 3 switches in the path do not reassemble the fragments. The destination
host performs the reassembly and places the fragments in the correct order by looking at
the identification and fragment offset fields.
If one or more fragments are lost, the entire packet must be retransmitted. Retransmission
is the responsibility of the higher-layer protocol (such as TCP). Also, you can set the Flags
field in the IP header to “Do Not Fragment” the packet. If the field indicates Do Not Frag-
ment, the packet is discarded if the outgoing MTU is smaller than the packet.
This section covers the IPv4 address classes, private addressing, and NAT. The IPv4 address
space was initially divided into five classes. Each IP address class is identified by the initial
bits of the address. Classes A, B, and C are unicast IP addresses, meaning that the destina-
tion is a single host. IP Class D addresses are multicast addresses, which are sent to multi-
ple hosts. IP Class E addresses are reserved. Private addresses are selected address ranges
that are reserved for use by companies in their private networks. These private addresses
are not routed in the Internet. NAT translates between private and public addresses.
An IP addresses is a unique logical number to a network device or interface. An IP address
is 32 bits in length. To make the number easier to read, the dotted-decimal format is used.
The bits are combined into four 8-bit groups, each converted into decimal numbers (for
example, 10.1.1.1). If you are not familiar with binary numbers, Appendix C contains a re-
view of binary and hexadecimal number manipulation.
The following example shows an IP address in binary and decimal formats:
Binary IP address: 01101110 00110010 11110010 00001010
Convert each byte into decimal.
For the first octet:
0
1
1
0
1
1
1
0
0
+64
+32
+0
+8
+4
+2
+0 = 110
01101110 = 110
For the second octet:
0
0
1
1
0
0
1
0
0
+0
+32
+16
+0
+0
+2
+0 = 50
00110010 = 50
