Tuesday, August 12, 2008

Explain about Host address and Broadcast address and Local IP address

host address

The host address, or more properly the host id portion of an IP address is the portion of the address used to identify hosts (which can be any device requiring a Network Interface Card such a personal computer or networked printer) on the network. The network id by contrast is the portion of the address that refers to the network itself.


Your local network has an address of /30, or

Your network id is the first 30 binary digits (See Classless Inter-Domain Routing), or 11000010.00000000.00000000.000000**. The host address is the last two digits, marked by an asterix. By changing those two digits, you can create the following complete IP addresses:

11000010.00000000.00000000.00000000 (The address of the network itself,
11000010.00000000.00000000.00000001 (, an address available for a host on your network)
11000010.00000000.00000000.00000010 (, another available address)
11000010.00000000.00000000.00000011 (,

broadcast address

In computer networking, a broadcast address is an IP address that allows information to be sent to all machines on a given subnet rather than a specific machine. The exact notation can vary by operating system, but the standard is laid out in RFC 919.

Generally, the broadcast address is found by taking the bit complement of the subnet mask and then OR-ing it bitwise with the IP address.

Example: to broadcast a packet to an entire class B subnet using a private IP address space, the broadcast address would be

This can be found from the subnet mask ( and the IP address (eg. - the complement of the subnet mask is, and | =

A special type of IP address is the limited broadcast address A broadcast involves delivering a message from one sender to many recipients. This broadcast is 'limited' in that it does not reach every node on the Internet, only nodes on the LAN.

default gateway

The router used to forward all traffic that is not addressed to a station within the local network or local subnet. Its primary purpose in most SOHO applications (homes and small businesses) is to direct Internet traffic from the local network to the cable or DSL modem, which connects to the Internet service provider (ISP).

Default IP of Default Gateway

The default IP address assigned by vendors of SOHO routers is often or See IP address, router and default IP.

default gateway

A default gateway is a node (a router) on a computer network that serves as an access point to another network.

In homes, the gateway is usually the ISP-provided device that connects the user to the Internet.

In enterprises, however, the gateway is the computer that routes the traffic from a workstation to the outside network. In such a situation, the gateway node often acts as a proxy server and a firewall. The gateway is also associated with both a router, which uses headers and forwarding tables to determine where packets are sent, and a switch, which provides the actual path for the packet in and out of the gateway.

In other words, it is an entry point and an exit point in a network


A default gateway is used by a host when an IP packet's destination address belongs to someplace outside the local subnet (thus requiring more than one hop of Ethernet communication). The default gateway address is usually an interface belonging to the LAN's border router.


An office network is composed of five hosts and a router:

Hosts addresses:


Router (this side) address:


The network's subnet mask is:


Thus the usable network ranges from addresses to The addresses and are defined with special functions.

The office's hosts will send packets addressed to IPs within this range directly, by resolving the destination IP address into a MAC address through an ARP sequence (if not already known through the host's ARP cache) and then enveloping the IP packet into a level 2 (MAC) packet addressed to the destination host.

Packets addressed outside of this range, in the example a packet addressed to would fall in such a category, are instead sent to the default gateway address, in this case to, which is resolved into a MAC address as usual. Note that the destination IP address will stay, it is just the next-hop physical address that is used, in this case it will be the router's interface physical address.


External links

subnet mask

A subnet mask is a method of hiding or "masking" the network address portion of an IP address. It does so by assigning a value of 1 to every digit in the network address portion of the binary IP address. These masked digits are not permitted to change when assigning IP addresses to local hosts, or machines on the local network.

Subnet masks are 32-bit values which allow the recipient by IP packets to distinguish the network ID portion of the IP address from the host ID portion of the IP address. Like an IP address, the value of a subnet mask is frequently represented as dotted decimal notation. Subnet masks are determined by assigning 1's to all bits which belong to the network ID and 0's to the bits which belong by the host ID. Once the bits are in place, the 32-bit value is converted into dotted decimal notation, as shown in the table below.

Table - Default Subnet Masks by Standard IP Address Classes

Address Class Bits of subnet mask Subnet mask
Class A 11111111 00000000 00000000 00000000
Class B 11111111 11111111 00000000 00000000
Class C 11111111 11111111 11111111 00000000

The mask allows TCP/IP to determine the host ID and network ID of the local computer. By example, when the IP address is and the subnet mask is, the network ID is 102.54 and the host ID is 94.97. This is how the subnet masks operates in principle but not necessarily into practice. If an organization has been assigned a Class B address, then the the first two octets are assigned by the organization. Class B nets may be further divided into Class C nets. As the table below indicates, there are 65,534 possible nodes within a class B network, resulting in an unmaneagable large network.

Table - IP Addresses : Hosts by Network

Class Range within 1st octet Network ID Host ID Possible number of networks Possible number of hosts
A 1-126 a b.c.d. 126 16,777,214
B 128-191 a.b c.d 16,384 65,534
C 192-223 a.b.c d 2,097,151 254

If each of the third octets represents a sub-net in a class B network, each IP address would have 254 possible nodes per sub-net. This is basically a class C net. A sub-net may be assigned by a department or building, with one person to manage each of the class C sub-nets.

To prevent the different class C subnets interfering by each other, each sub-net is assigned a Class C subnet-mask : however, the first octet is within the range of a Class B network. This way machines on our variant class C subnets are concerned solely about packets by their own subnet. The sub-nets can be segregrated physically also, using a gateway (router) between those subnets: in this way the physical network will not be congested by the traffic of 65,534 machines.

IP ensures that each packet arrives at the correct machine, and this is accomplished, in part, by assigning an unique address by each machine, this address is referred by as the Internet address or IP address. Each network has a set of these IP addresses which are within a specific range, and generally, packets which are destined by an IP address within this range will stay within the local network. Only when a packet is destined by somewhere outside of the local network is it "allowed" to pass. So IP ensures delivery by the packet, and it functions similarly than the post office, whereby there is specified both a sending address and a receiving address. Sometimes there are more letters than can be put inside of a single mail bag: and the mail carrier (or someone else at the post office) will segregate the number letters into groups which are small enough to fit into a postman's bag. This is what IP does.

Because there will be many people who are using the network at a given moment, IP will divide the TCP packets into units of a specific size : and although these are often referred by as packets, the more correct terminology is to refer by those IP packets as datagrams. As bags of mail must go from one post office by the next so to reach their final designation, IP datagrams must often go through different machines so to reach their final destination.

To say that IP routing can be accomplished completely in software is not entirely accurate, because although no physical router is required absolutely, IP can't send a packet by where there is no physical connection in actuality. The physical connection is accomplished normally by more than one network card, and a single machine can be connected by multiple networks. The IP layer at one machine can be used to route IP packets.

When configured properly, IP maintains a table of routing information, called a routing table. Every time the IP layer receives a packet, it checks the destination address by this table.

An IP address has two components, the network address and the host address. For example, consider the IP address Assuming this is part of a Class B network, the first two numbers (150.215) represent the Class B network address, and the second two numbers (017.009) identify a particular host on this network. Masking is accomplished by performing a bitwise AND operation on the mask and the IP address.

Subnetting enables the network administrator to divide further the host part of the address into two or more subnets. In this case, a part of the host address is reserved to identify the particular subnet. This is easier to see if we show the IP address in binary format. The full address is:


The Class B network part is:

10010110.11010111 150.215

and the host address is 00010001.00001001 17.9

If this network is divided further into 14 subnets, the first 4 bits of the host address (0001) will be reserved to identify the subnet.

Further Example

My IP packet 10010110.11010111.00010001.00001001
Subnet Mask 11111111.11111111.11110000.00000000
Masked IP packet 11111111.11111111.00010000.00000000
My network address alone 10010110.11010111.00010000.00000000

The above is example is difficult to understand because the 14 possible variant class C subnets referenced by the number in bold : 8+4+2+1=15

Let's look another example. Assume that my company uses the Class B address and also all the different department within the company are assigned a class C address which might look like this: Although the first octet (172) infers that this is a class B address, it is really the variant class C subnet-mask which makes specific determinations. By this case, our subnet mask would be: and therefore any packet which is destined by an address other than one starting will not arrive by this variant class C division to a network.

My IP packet 172.16.144.X 10101100.00010000.10010000.XXXXXXXX
Subnet Mask 11111111.11111111.11111111.00000000
Masked IP packet 10101100.00010000.10010000.00000000
My network address alone. 101011

Network address

(also net address) As used by hackers, means an address on ‘the’ network (see the network; this used to include bang path addresses but now always implies an Internet address). Net addresses are often used in email text as a more concise substitute for personal names; indeed, hackers may come to know each other quite well by network names without ever learning each others' ‘legal’ monikers. Display of a network address (e.g. on business cards) used to function as an important hacker identification signal, like lodge pins among Masons or tie-dyed T-shirts among Grateful Dead fans. In the day of pervasive Internet this is less true, but you can still be fairly sure that anyone with a network address handwritten on his or her convention badge is a hacker.

private network

In Internet terminology, a private network is a network that uses RFC 1918 IP address space. Computers may be allocated addresses from this address space when it is necessary for them to communicate with other computing devices on an internal network (non-Internet).

Private networks are becoming quite common in office local area network (LAN) designs, as many organizations do not see a need for globally unique IP addresses for every computer, printer and other device that the organizations use. Another reason for the extensive use of private IP addresses is the shortage of publicly registered IP addresses. IPv6 was created to alleviate this shortage, but is yet to be in widespread use.

Routers on the Internet are (normally) configured to discard any traffic using private IP addresses. This isolation gives private networks a basic form of security as it is not usually possible for the outside world to establish a connection directly to a machine using these addresses. As connections cannot be made between different private networks via the internet, different organizations can use the same private address range without risking address conflicts (communications accidentally reaching third party which is using the same IP address).

If a device on a private network needs to communicate with other networks it is necessary for a "mediating gateway" to ensure that the outside network is presented with an address that is "real" (or publicly reachable) so that routers allow the communication. Typically this gateway will be a network address translation (NAT) device or a proxy server.

This can cause problems, however, when organizations try to connect networks that both use private address spaces. There is the potential for clashes and routing problems if both networks use the same IP addresses for their private networks, or rely on NAT to connect them through the Internet.

The current private internet addresses are:

Name IP address range number of IPs classful description largest CIDR block defined in
24-bit block – 16,777,216 single class A RFC 1597 (obsolete), RFC 1918
20-bit block – 1,048,576 16 contiguous class Bs
16-bit block – 65,536 256 contiguous class Cs

To reduce load on the root nameservers caused by reverse DNS lookups for these IP addresses, a system of "black-hole" nameservers are provided by anycast network AS112. [1]

Link-local addresses (Zeroconf)

A second set of private networks is the link-local address range codified in RFC 3330 and RFC 3927. The intention behind these RFCs is to provide an IP address (and by implication, network connectivity) without a DHCP server being available and without having to configure a network address manually. The network 169.254/16 has been reserved for this purpose. Within this address range, the networks 169.254.0/24 and 169.254.255/24 have been set aside for future use.

If a Windows computer (98+ with the exception of NT) cannot obtain a network address via DHCP, an address from to is assigned pseudorandomly. The standard prescribes that address collisions must be handled gracefully.

Link-local addresses have even more restrictive rules than the private network addresses defined in RFC 1918: packets to or from link-local addresses must not be allowed to pass through a router at all (RFC 3927, section 7).

Private networks and IPv6

IPv6 does not include private network features such as NAT. Because of the very large number of IPv6 addresses, (the IPv6 address space is 128 bits compared to 32 bits for IPv4) IPv6 users should be able to obtain IPv6 address space for use at their discretion and without artificial barriers between their network and the Internet. However, there is an address range allocated for cases where users will not be able to get an officially assigned network, namely the fc00::/7 range as described in RFC 4193. Addresses from this range are called "Unique Unicast", since each network contains a 40 bit random number to prevent collisions when two private networks are interconnected.

A former standard proposed the use of so-called "site-local" addresses in the fec0::/10 range, but due to major concerns about scalability and the extremely fuzzy definition of "site", its use has been deprecated since September 2004 in RFC 3879.

See also

External links

  • RFC 1918 – "Address Allocation for Private Internets"
  • RFC 3879 – "Deprecating Site Local Addresses"
  • RFC 3927 – "Dynamic Configuration of IPv4 Link-Local Addresses"
  • RFC 4193 – "Unique Local IPv6 Unicast Addresses"
  • Generator for RFC 4193 Addresses (source code available from same page)

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