39
If a DHCP request
has…
Handling
strategy
DHCP snooping…
No Option 82
N/A
Forwards the message after adding the Option 82 padded
according to the configured padding format, padding
content, and code type.
IP
IP address classes
IP addressing uses a 32-bit address to identify each host on an IPv4 network. To make addresses
easier to read, they are written in dotted decimal notation, each address being four octets in length.
For example, address 00001010000000010000000100000001 in binary is written as 10.1.1.1.
Each IP address breaks down into the following sections:
•
Net
ID
—Identifies a network. The first several bits of a net ID, known as the class field or class
bits, identify the class of the IP address.
•
Host
ID
—Identifies a host on a network.
IP addresses are divided into five classes. The following table shows IP address classes and ranges.
The first three classes are most typically used.
Class Address
range
Remarks
A
0.0.0.0 to 127.255.255.255
The IP address 0.0.0.0 is used by a host at startup
for temporary communication. This address is
never a valid destination address.
Addresses starting with 127 are reserved for
loopback test. Packets destined to these
addresses are processed locally as input packets
rather than sent to the link.
B
128.0.0.0 to 191.255.255.255
N/A
C
192.0.0.0 to 223.255.255.255 N/A
D
224.0.0.0 to 239.255.255.255 Multicast
addresses.
E
240.0.0.0 to 255.255.255.255
Reserved for future use, except for the broadcast
address 255.255.255.255.
Subnetting and masking
Subnetting divides a network into smaller networks called subnets by using some bits of the host ID
to create a subnet ID.
Masking identifies the boundary between the host ID and the combination of net ID and subnet ID.
Each subnet mask comprises 32 bits that correspond to the bits in an IP address. In a subnet mask,
consecutive ones represent the net ID and subnet ID, and consecutive zeros represent the host ID.
Before being subnetted, Class A, B, and C networks use these default masks (also called natural
masks): 255.0.0.0, 255.255.0.0, and 255.255.255.0, respectively.
Subnetting increases the number of addresses that cannot be assigned to hosts. Therefore, using
subnets means accommodating fewer hosts.
