Special Case Ip Addresses; Subnetting And Masking - 3Com WX3000 Series Operation Manual

Table 1-1 IP address classes and ranges
Class
A 0.0.0.0 to 127.255.255.255
B 128.0.0.0 to 191.255.255.255
C 192.0.0.0 to 223.255.255.255
D 224.0.0.0 to 239.255.255.255
E 240.0.0.0 to 255.255.255.255

Special Case IP Addresses

The following IP addresses are for special use, and they cannot be used as host IP addresses:
IP address with an all-zeros net ID: Identifies a host on the local network. For example, IP address
0.0.0.16 indicates the host with a host ID of 16 on the local network.
IP address with an all-zeros host ID: Identifies a network.
IP address with an all-ones host ID: Identifies a directed broadcast address. For example, a packet
with the destination address of 192.168.1.255 will be broadcasted to all the hosts on the network
192.168.1.0.

Subnetting and Masking

Subnetting was developed to address the risk of IP address exhaustion resulting from fast expansion of
the Internet. The idea is to break a network down into smaller networks called subnets by using some
bits of the host ID to create a subnet ID. To identify the boundary between the host ID and the
combination of net ID and subnet ID, masking is used.
Each subnet mask comprises 32 bits related to the corresponding bits in an IP address. In a subnet
mask, the part containing consecutive ones identifies the combination of net ID and subnet ID whereas
the part containing consecutive zeros identifies the host ID.
Figure 1-2 shows how a Class B network is subnetted.
Figure 1-2 Subnet a Class B network
Class B address
Mask
Subnetting
Mask
While allowing you to create multiple logical networks within a single Class A, B, or C network,
subnetting is transparent to the rest of the Internet. All these networks still appear as one. As subnetting
Address range
0 7
Net-id
1 0
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
Net-id
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0
Address 0.0.0.0 means this host no this network.
This address is used by a host at bootstrap when it
does not know its IP address. 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.
––
––
Multicast address.
Reserved for future use except for the broadcast
address 255.255.255.255.
15 23
Host-id
Subnet-id Host-id
1-2
Remarks
31