40
For example, a Class B network without subnetting can accommodate 1022 more hosts than the
same network subnetted into 512 subnets.
•
Without
subnetting
—65534 (2
16
– 2) hosts. (The two deducted addresses are the broadcast
address, which has an all-one host ID, and the network address, which has an all-zero host ID.)
•
With
subnetting
—Using the first nine bits of the host-id for subnetting provides 512 (2
9
)
subnets. However, only seven bits remain available for the host ID. This allows 126 (2
7
– 2)
hosts in each subnet, a total of 64512 (512 × 126) hosts.
IP address configuration methods
You can use the following methods to enable an interface to obtain an IP address:
•
Manually assign an IP address to the interface.
•
Configure the interface to obtain an IP address through DHCP.
MTU for an interface
When a packet exceeds the MTU of the output interface, the device processes the packet in one of
the following ways:
•
If the packet disallows fragmentation, the device discards it.
•
If the packet allows fragmentation, the device fragments it and forwards the fragments.
Because fragmentation and reassembling consume system resources, set an appropriate MTU for
an interface based on the network environment to avoid fragmentation.
ARP
ARP resolves IP addresses into MAC addresses on Ethernet networks.
Types of ARP table entries
An ARP table stores dynamic and static ARP entries.
Dynamic ARP entry
ARP automatically creates and updates dynamic entries. A dynamic ARP entry is removed when its
aging timer expires or the output interface goes down. In addition, a dynamic ARP entry can be
overwritten by a static ARP entry.
Static ARP entry
A static ARP entry is manually configured and maintained. It does not age out and cannot be
overwritten by any dynamic ARP entry.
Static ARP entries protect communication between devices because attack packets cannot modify
the IP-to-MAC mapping in a static ARP entry.
The device supports the following types of static ARP entries:
•
Long static ARP entry
—It contains the IP address, MAC address, VLAN, and output interface.
It is directly used for forwarding packets.
•
Short static ARP entry
—It contains only the IP address and MAC address.
{
If the output interface is a VLAN interface, the device sends an ARP request whose target IP
address is the IP address in the short entry. If the sender IP and MAC addresses in the
received ARP reply match the short static ARP entry, the device performs the following
operations:
