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Networks provide communication between computing devices. To communicate properly, all computer (hosts) on a network need to use the same communication protocols. An Internet Protocol network is a network of computer using Internet Protocol for their communication protocol.

All computer within an IP network must have an IP address that uniquely identifies that individual host. An Internet Protocol-based network (an IP Network) is a group of hosts that share a common physical connection and that use Internet Protocol for network layer communication. The IP addresses in an IP network are contiguous, that is, one address follows right after the other with no gaps.

Address Types in an IP Network

Within a given range of IP addresses used in every IP network are special addresses reserved for

• Host Addresses
• Network Addresses
• Broadcast Addresses

In addition, a IP network has a subnet mask. The subnet mask is a value stored one each computer that allows that computer to identify which IP addresses are within the network to which they are attached, and which IP addresses are on an outside network.

Host address

A host's IP address is the address of a specific host on an IP network. All hosts on a network must have a unique IP address. This IP address is usually not the first or the last IP address in the range of network IP addresses as the first IP address and last IP address in the range of IP addresses are reserved for special functions. The host addresses are all the addresses in the IP network range of IP addresses except the first and last IP addresses. Host IP addresses allow network hostst to establish one-to-one direct communication. This one-to-one communication is referred to as unicast communication.

All host IP addresses can be split into two parts: a network part and a host part. The network part of the IP addresses identifies the IP Network the host is a member of. The host part uniquely identifies an individual host.

Network Address

The network address is the first IP address in the range of IP addresses. To be more precise, the network address is the address in which all binary bits in the host portion of the IP address are set to zero. The purpose of the Network Address is to allow hosts that provide special network services to communicate. In practice, the network address is rarely used for communication.

Broadcast Address

The broadcast IP address is the last IP address in the range of IP addresses. To be more precise, the broadcast address is the IP address in which all binary bits in the host portion of the IP address are set to one. The broadcast address is reserved and allows a single host to make an announcement to all hosts on the network. This is called broadcast communication and the last address in a network is used for broadcasting to all hosts because it is the address where the host portion is all ones. This special address also sometimes called the all hosts address. Some vendors allow you to set an address other than the last address as the broadcast address.

Note that each network has its own network and broadcast addresses. Network addresses do not necessarilly have a zero in the decimal representation of the host portion of their address.

EXAMPLE 1

Let's take the example of an IP network that has an address range of 192.168.1.0 - 192.168.1.255 with an address mask of 255.255.255.0, which would be 192.168.1/24 in CIDR block notation:

HINT: If this table is giving you trouble, you might need help with binary.

EXAMPLE 2

Example 1 showed a standard "Class C" style range of addresses where the entire last octet of the IP address is used for one network. Some networks have fewer hosts and conservation of IP addresses is important, so using a subnet mask reduces the number of hosts in a range of IP addresses on an IP network.

In example 2, we borrow 2 bits from what would normally be the host portion and use it as bits that indicate the network portion. This makes a smaller network of just 64 addresses, of which 62 are usable for hosts. Remember, the first address in a range of IP addresses is reserved for the network address. The last address in the IP address range for the network is reserved for the broadcast address.

CAUTION:The CCNA exam will try to trip you up by asking how many addresses are usable for hosts, which is always 2 less than the number of addresses in the range of addresses in the subnet. Read your subnetting questions carefully on the CCNA exam and understand whether they are talking about the range of addresses, the number of host addresses, the number of networks, the network address or the broadcast address.

192.168.1.0 / 255.255.255.192 (192.168.1/26)

HINT: If this table is giving you trouble, you might need help with binary.

HOW IT WORKS

Your computer uses a a mask in its network connection that allows it to determine whether the computer it wants to talk to over the local network is actaully on the local network or on a network outside the local network. If it is a computer that is local, it will use a local protocol to find the other computer's hardware address. If the other computer is not local (it is outside the local network), then it will send the data to the local gateway (usually called the default gateway). Your computer will try to determine the hardware address of the local default gateway and will use the hardware address in the Ethernet header (or FDDI header or Token Ring header etc.). The Ethernet frame will contain a payload which will be filled with IP data. The IP address of the remote computer will be in this payload as part of the IP header. Since the IP address in the IP datagram is the remote computer, but the Ethernet frame is addressed to the local default gateway, the default gateway will accept the Ethernet frame, read the IP data, verify the IP address is indeed outside the local network and then forward the IP datagram over whatever other network connections the computer has that will allow it to reach all the other outside networks.

That's how Internet Protocol makes routing work in a network with a default gateway.

Fact and Fiction

Routers are NOT used for local communication on the local IP network. They are bypassed completely and are not required. Routers are used to forward data into different networks and to restrict broadcasts on the local network from swamping external networks with traffic that is important only to the local network.