3Com WX3000 Special Case IP Addresses, Subnetting and Masking, Class Address range Remarks

Table 1-1IP address classes and ranges

Class	Address range	Remarks
		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	0.0.0.0 to 127.255.255.255	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	––
C	192.0.0.0 to 223.255.255.255	––
D	224.0.0.0 to 239.255.255.255	Multicast address.
E	240.0.0.0 to 255.255.255.255	Reserved for future use except for the broadcast
		address 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:

zIP 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.

zIP address with an all-zeros host ID: Identifies a network.

zIP 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 1-2Subnet a Class B network

0

Class B address 1 0

7	15	23	31
Net-id		Host-id

Mask 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

Subnetting

Mask

Net-id Subnet-id Host-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

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

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