Category Archives: Network Layer

Broadcast Address

A broadcast address is defined as a special address used by all the devices in a network. When a specific packet is addressed to the network, the said devices use the broadcast address to open and process the message contained in the packet.

Through the broadcast address, any device in the network can see the message, open it, and pass it on to the next workstation. Also, through the broadcast address, the network administrator can ensure that the needed information will be delivered to every client in the network. Thus, no workstation is excluded in the sharing of information. The broadcast address cannot be assigned as an IP address to any computer in the network.

The exact notations used in broadcast addresses differ from one Operating System to another. The standard notation, however, is indicated in RFC 919. RFC (Request for Comments) is a memorandum created by the Internet Engineering Task Force.

A subnet refers to a portion of a network physically independent from the main network. In a subnet environment, the broadcast address has the highest numerical value in the subnet’s range of numbers.

A broadcast address is determined through a set of processes. Initially, the bit complement of the subnet mask is taken. Then, the bit complement goes through a process known as “or-ing bitwise”, which is carried out using a specific IP address. Once these processes are performed, the message is relayed to the stations in the network. This is done by passing the information up through each network layer.


Routing refers to the process of transferring packets of data through a network or an interconnection of networks. The Internet is the most well-known example of a system where routing is applied.

Routing is a process which operates within the OSI model’s network layer. This is in contrast to other similar processes known as switching and bridging. The said processes occur on the data link layer.

Routing involves a number of tasks. First is the process of defining the paths for the moving of data packets through the interconnection of networks. In this process, the source and the target destination are identified. The other nodes which the packet may go through may be identified as well.

The second step entails the forwarding of the packets based on the determined paths. This is the final process in routing. Here, data is sent out by the source and is passed through the network until it reaches the destination. Depending on the network settings, other nodes may be informed that the transfer is being done. The target will provide a confirmation when the packet of data has been received successfully.

Routing has two major types: static and dynamic routing. In static routing, the information needed by packets to reach their destination is entered manually. Static routing requires the updating of the routing table whenever the network topology changes. This type of routing is ideal for smaller networks.

Dynamic routing is an automatic means of moving packets through networks. Dynamic routing uses a set of protocols in creating and managing the routing tables. Hence, dynamic routing is more responsive to network changes and even failures.

IP Address Conflict

What is an IP address?

An Internet Protocol (IP) address is a logical address or a numerical identification assigned to a device participating in a computer network. This unique identification basically names the device, locates it, and defines the route on how to get there.

An IP address may be private, for use in a Local Area Network (LAN), or public, for use on the Internet or other Wide Area Network (WAN). It is usually assigned by a statically determined system administrator or assigned by another dynamically determined device on the network.

What is an IP Address Conflict?

IP address conflicts happen when two computers on the Internet or in a Local Area Network have the same IP address. An IP conflict render either one or both of the computers unusable because of intermittent connection or it cuts off the connection all together.

How Does an IP Address Conflict Happen?

An IP address conflict may happen through any of the following:

  • A system administrator statically assigns an IP address to two computers in a Local Area Network;
  • A system administrator statically assigns a computer with an IP address within a LAN’s DHCP range, while a LAN DHP server automatically assigns the same address;
  • Multiple computers are automatically assigned with the same dynamic address due to a malfunction in the DHCP’s server;
  • Two customers are accidentally assigned with the same dynamic or static address by their Internet service providers (ISP);
  • A malfunction caused by a system coming back online from an extended period in stand-by or hibernate mode;
  • A computer configured with multiple network adaptors; or
  • A system administrator connects two ports of a network switch or a router to each other.

How Do You Recognize an IP Address Conflict?

Most Microsoft Windows computers will show this pop-up error message when attempting to set-up a static IP address already active in a local network:

The configured static IP address is already in use on the network. Please reconfigure a different IP address.

Conversely, Microsoft Windows computers with dynamic IP conflicts will receive a balloon error message in the taskbar once the Operating System detects it. It will display the following message:

“There is an IP address conflict with another system in the network.”

A similar message may appear in a pop-up window in computers with older versions of Microsoft Windows:

“The system has detected a conflict for IP address…”

The same message may appear in Mac or Linux computers.

How is an IP Address Conflict Resolved?

IP address conflicts are most likely to resolve themselves in DHCP environments as the systems request for valid addresses. However, this process may take a while or it may not even resolve itself.

The following procedures may help resolve IP address conflicts:

  • Computers with statically assigned IP addresses can resolve this issue by ensuring that each local host is configured with a unique IP address.
  • Computers with dynamically assigned IP address can resolve the issue through the following steps:

1. Release and renew its IP address by manually entering IPCONFIG /RELEASE and IPCONFIG /RENEW from a command prompt.

2. If the conflict still persists, the system may have been issued with the same address and another system in the network may be the source of the problem. The offender can be traced by checking the System Log within the Windows Event Viewer.


3. Look for a DHCP ERROR ENTRY by running IPCONFIG/ ALL on a Windows system or by checking the label on the back or underside of a non-PC device. The DHCP error entry will list a network address that can be used to find the conflicting device.

Releasing and renewing an IP address or starting the other device should resolve the conflict. However if the problem persists, restart the router to reset the DHCP server and force all systems to request for new IP addresses.

  • Systems experiencing chronic IP address conflicts with no traceable cause should consider updating the router with its most recent firmware.

For more information on IP Address Conflict read:

  • IP Address Conflict
  • DHCP Reservation

    A DHCP (Dynamic Host Configuration Protocol) reservation means that a permanent ID is leased to a client, hardware or server. Configuring a DHCP reservation is recommended for a client who needs a static address.

    Reservations can be used with DHCP-enabled equipment such as print, file, and other application servers because these are applied with the permanent IP address in the network.

    The Reservation

    A reservation generally consists of detailed information regarding the client. The necessary elements needed for input are: a reservation name, which the administrator allocates for the client; the IP address, which must be taken from the scope or “pool”; and the client’s MAC (Media Access Control) address, entered without the hyphens and with a description of the reservation account.

    Client Verification

    The IP address of a client can be determined by viewing the TCP/IP properties in his computer.

    • The ipconfig/all command will verify if the DHCP server IP address given is the real IP address of the partner network connection.
    • The ipconfig/release command is used to release the IP address from the server.
    • The ipconfig/renew command is used to renew the IP address from server.

    Some companies prefer this set up, because creating DHCP reservations for each client machine on the network makes the administrator’s work easier. This way, every time a client gets a lease on an address using DHCP, it gets the same IP address out of the DHCP server.

    The administrator does not have to work on every machine in the network and configure on each of the IP address settings manually. Instead, it can be done centrally on the DHCP server, provided that the MAC addresses of each client on the network is readily available. There is a big advantage in using an all-reserved approach. It provides better security because each client always uses the same IP address. So, it is effortless to interpret the address information posted in the firewall logs.

    Finally, a DHCP reservation helps to ensure that client machines can be easily traced in terms of Internet usage per user, if the company has this policy.


    DHCP stands for Dynamic Host Configuration Protocol. It is used to allocate IP addresses to a network’s work stations or hosts. A common tool known as DHCP server performs this allocation.

    What DHCP does is to “lend” or “lease” addresses to different hosts on particular times. Whenever a host is not able to use a certain IP address for a period of time, such address will be “lent” and allocated to another unit determined by the DHCP server.

    DHCP works by using Ethernet (for MAC) or any hardware address in order to establish the IP address allocations. DHCP protocol is founded on BOOTP; therefore, DHCP actually replaces and extends BOOTP.

    The Lease Stages of DHCP

    1. The first lease stage of DHCP is the lease request. It is during this stage where a client requests for an IP address through a broadcast.
    2. The second stage (Lease Offer) takes the form of a broadcast message (DHCPOFFER). By then, the requested IP address is blocked from other clients and is made available to the one client it is being offered to.
    3. Should the lease offer be accepted by the client, a DHCPREQUEST message is sent to the service provider.
    4. At this point, the fourth stage, or the lease acknowledgment, takes place. The server sends a DHCPNACK or a DCHPACK if the client requested an unavailable address. However, it is not a permanent status. The client has to renew the lease after 50% of the lease time has expired. If it has passed the 50% mark and has progressed to the 87.5% mark, the client then attempts to contact any other DCHP server. Should this fail and the lease expires, the TCP/IP address will cease to function.


    Exclusion ranges exist in order to reserve a bank of IP addresses so that computers needing to use specific IP addresses will be able to access them. This is because client reservation works by making sure a computer gets the same IP address every time it renews or acknowledges a lease.


    It is important to take note of the configuration coding system of a DCHP because it is where the time limit is stated. Not using the IP address within the given time means the address will be released and given to other computers.

    Subnet Mask

    A subnet mask is an address composed of 32-bits expressed in numbers. It operates to identify the network address in an IP address. It is also intended to segregate the IP address into sub-categories like the host and network address.

    A subnet mask determines the magnitude of a subnet. It also helps identify the source on the subnet whenever the user is given an IP address. It is referred to as a subnet “mask” because it literally masks away all the host bits. It reveals only the Network ID that launches the subnet.

    The Host Address

    The host is normally configured with an exclusive IP address and a subnet mask assigned to each interface that occurs on the network. Mostly, the host will possess only one network interface, which is why one IP address and one subnet mask is applied to it.

    Hosts obtain the IP address and the subnet mask from DHCP(Dynamic Host Configuration Protocol). They may also be manually installed in the computer by the network administrator.

    Why it is Needed

    Using a subnet mask will define the IP subnet boundaries. Likewise, the interchange between the IP address and the subnet mask series adheres to a defined mathematical formula. Thus, the IP address will be a transparent identity, and the reserved and available parts can be seen by the host. Each network is usually configured with one IP address and subnet mask installed on a part of a network-capable calculator.

    How the Mask Works

    Every valid subnet mask consists of two parts:

    1. The left side including all the mask bits set to ‘1’ is the extended network portion.

    2. The right side including all mask bits, also set to ‘0’ is the host portion.

    The existing computer network is divided by the mask into smaller networks or “subnets.” The effects of this splitting can be seen in the reduction of the broadcast domains because of lesser traffic. It also enables LANs of far distances to connect and communicate with each other. Moreover, the security aspect of the computer must be taken into account. Masking avoids the overlapping of LAN with one another.

    It does not work like an IP address nor does it stay independently from that address. Instead, the subnet mask goes along with an IP address and works together with it.

    Additional Reading on Subnet Mask


    A sub-network (subnet) is a smaller network under a bigger one. It gives an immediate identification as to where the information originates. A subnet allows the good flow of network transmissions among host sites. These sites or networks can be segregates using a configuration called a subnet pattern.

    The Subnet

    A subnet may typically stand for the machines located in the same Local Area Network or the LAN. The computer can share a single network address when accessing the Internet.

    Installing a subnet involves splitting a Transmission Control Protocol/Internet Protocol (TCP/IP) into smaller, more manageable pieces.

    The idea is that a network with a huge amount of transmission flowing through its system initiates a traffic condition. When congestion happens, the network tends to slow down. When a subnet is applied to the network, the entire network is split into smaller interconnected parts that work as one system.

    Why Use a Subnet?

    Many benefits can be derived from the implementation of a subnet system. Application of subnet in an IP Network is done for the organization and the use of physical network media like WAN, Ethernet, and FDDI.

    The common purpose of subnetting is to manage network traffic. A subnet will reduce congestion which can reach a global extent. It unloads a big part of work from the Internet administrator. This results in a faster system. The work will be decentralized and the network will flow with ease.

    Another benefit is the conservation of address space and security. Without the subnet, a work group may make multiple connections with the Internet and impose an unnecessary task of assigning the bulk of addresses. Each machine is an individual network and this will involve the use of the limited network numbers the Internet needs to assign. It also requires the Internet gateway to manage the routing that should be handled by the organization. The result is much activity on the gateway outside and a higher traffic. However, this can be avoided when a company takes the initiative to install subnets in the workplace.

    MAC Address

    The Media Access Control (MAC) Address is a unique value that serves as an identifier of network adapters (Network Interface Card). A MAC address exists on Layer 2 of the Open Systems Interconnection (OSI) model. It uniquely recognizes an adapter on a Local Area Network (LAN) or Metropolitan Area Network (MAN).

    The OSI model is a reference model used to describe the layered communications design and network applications. The Layer 2, out of the 7 layers of the OSI model, is the Data Link Layer. It consists of two sub-layers: the Logical Link Control (LCC) and the MAC layer. The MAC sublayer controls how a network node gets an access to the data and the approval to send the data. Together, they enable computers to distinctively recognize themselves on a network at this comparatively low level.

    A MAC address is also known as physical addresser hardware address. At the time of manufacture of the hardware, a MAC address with a globally unique value is written into the hardware. Because of this, a MAC address is at times called Burned-In Address (BIA). In many cases, it is possible to change the MAC address of a device once the software is manufactured.

    A MAC address is composed of 12-digit numbers (48 bits in length). The first half of a MAC address represents the adapter manufacturer’s ID number regulated by an Internet standard body. The other half contains the serial number assigned by the manufacturer to the adapter. There are more than 281 trillion possible MAC addresses.

    MAC addresses are used by several Layer 2 technologies such as Ethernet, Bluetooth, Fibre Channel, ATM, 802.11, Token Ring, SCSI, and FDDI. MAC addresses originated in the Ethernet specification. This is why MAC addresses are also referred to as Ethernet addresses.

    In computer networking, a MAC address is every bit as important as an IP address. While MAC addresses function on the Data Link layer (Layer 2), IP addresses work on the Network Layer (Layer 3). The Network Layer controls the paths for data transfer through the network.

    MAC addresses support hardware implementation of the network stack. IP addresses support the software implementation. MAC addresses follow the network device and typically remain fixed. IP addresses change as the network device moves from one network to another.


    Subnetting refers to the separation of an IP network into multiple and smaller networks to reorganize and secure a system. It is used to break a network into smaller parcels, enabling these separate parcels to work as efficiently as possible.

    The IP Address

    The Internet itself is an assortment of networks, where each network has users communicating with one another. Each of these users bears the address of both the source and destination groups. The address also carries information about the machine involved in the network. This network is constantly associated with its user because it is the host computer on the other end. This is called the Internet Protocol (IP) address.

    The IP address has 32 bits. It is stated in numbers, where one part declares the information about the network and the other gives the distinct identity of the host in the network. Each number allocated to a host within the network can be noticed in clusters of four. The numbers are also separated from each other by a dot. A number can range from 0, the lowest to 255, the largest to be seen in the IP address.

    How it Works

    Every host is called a node. In the subnet, you will see all packets being transmitted by any node within the network. The performance of the network can be adversely affected when there is a bulky transmission along the circuits. There will be heavy traffic, collisions, and retransmissions which will tend to clog the pathways.

    Subnetting Eliminates Traffic

    The central objective of implementing subnetting is to facilitate relief in network congestion. Congestion was a greater setback for the network users before the creation of subnets.

    When the interchange of information intensifies, the network’s performance begins to undergo difficulties. This results to heavy traffic. One way to disentangle the problem of collision is through splitting the big network to isolated and smaller pieces.

    Subnetting is one of the most successful techniques in networking. The result is a segregated network with smaller parts which are still interconnected with each other. Users in the subnet will experience free-flowing communication and interchange. Traffic will be secluded only to that subnet where it originated.

    DHCP Scope

    A DHCP (Dynamic Host Configuration Protocol) scope refers to a valid series of IP addresses are offered for assignments or for lease to the client computers of a subnet. Scopes establish which of the IP addresses are granted to the clients. However, these scopes are first defined and activated prior to the use of DHCP clients.

    How to Create a Scope

    There are some pieces of information needed to create a new scope. You must use the DHCP console to construct a new scope. The list of requirements includes:

    • a scope tag or identity to name the particular scope;
    • a distinctive subnet mask to determine the IP address in the network;
    • a lease duration period or the length of time that a client can use the IP address; and
    • a list of all IP addresses covered within the scope.

    The Lease Duration

    An organization can have many IP addresses available. They usually have configurations that are seldom adjusted. The administrator can increase the duration of lease to lessen the incidence of lease renewal links between the DHCP server and the clients. These incidences are due to network traffic. DHCP-related activities can be reduced when DHCP clients have frequent renewal of their leases.

    In the case of frequent client movements in and out of the network, the administrator can also make a limited number of IP addresses. The lease duration can also be reduced so the addresses can be returned to the scope for reassignment to other clients.

    Other Considerations

    It is quite probable to give a configuration on a DHCP client with an infinite lease period. However, this must be employed with watchful attention. Even considerably stable settings contain an undeniable number of customer turnover.

    Considering the situation at the minimum level, you might consider adding or removing computer units from one area to another area. You can also make an allowance by using network adapters. In case this happens, the DHCP server must be notified of the removal of the lease so the IP address can be used to service other clients. If not, the IP address can remain unused. To avoid this occurrence, it is better to opt for reservation rather than make an infinite configuration.

    DHCP Relay

    DHCP is the acronym for Dynamic Host Configuration Protocol, which uses a relay system to communicate with the DHCP server. It functions as a proxy for the DHCP broadcast messages that need to be routed to remote sections in the subnet. The DHCP Relay is usually configured with the address of the DHCP server where the message should be relayed.

    The Relay Agent

    The Relay Agent is a program used to transmit DHCP messages between servers and clients situated on different subnets. It also enables the user to position the DHCP servers and clients on separate networks. Setting DHCP within a segment of network is simple. Every DHCP message is considered as an IP broadcast message. As a result, every computer included in the segment can actually listen and give its response to these broadcasts.

    The Relay Agent at Work

    When the DHCP messages are broadcast in one of the physical interfaces, the Relay Agent works to transmit these messages to other subnets included in the network.

    When a request is made by a client, the Relay Agent forwards the request to the servers specified in the list. If the DHCP server makes a reply, this reply is also uni-cast in the network.

    The Relay Agent includes information on the IP address and presents some policies to client requests, then forwards it to the server. It also takes out all pieces of information from the replies given by the server to the client.

    The Relay Commands

    The Relay Agent can be initiated by using the dhrelay command and typing the server name where the request is to be sent. The following options can be used to start it:

    • The command “I” will specify the interfaces in the network to be configured. If there is no particular interface indicated, all the network’s interfaces will be included. This will possibly eliminate the non-broadcast interface.
    • The command “p” will specify the port where the dhrelay will listen. The Relay Agent will transmit the request to the given servers on this port and will send responses to the clients using a bigger port.
    • Typing the command “d” will cause the Relay Agent to be always in the foreground, while the “q” command will disable the printing of the network’s configuration of dhrelay. is an address in the special class of IP addresses known as loopback addresses. In fact, any IP address that begins with 127 is a loopback address.

    Network packets sent to this address are not transmitted over wire; rather, these are received directly by the same computer as input. For this network address, therefore, there is no hardware involved, just pure software.

    Use for Testing and Debugging

    The localhost (another name for is used mainly for testing and debugging network software on a single computer. Furthermore, programs using inter-process communication use this address for communicating with processes on the same computer, without regard to the computer’s external IP address.

    Very often, the localhost is used for testing Web applications. A client (such as a Web browser) sends a request to a Web server residing on the same machine, which then responds to the request.

    Illegal Address for a Network Device

    No network device can have an address with a first octet of 127. Also, packets meant for the localhost should never appear on a network, nor should any interface forward any such packet. The loopback address must not appear as the source address for a packet sent outside a single node.

    Testing Using the ping Command

    Here’s how you can test the loopback address on your computer. At the command prompt, type in “ping” and press Enter. If you get a response such as “Reply from…,” then your system’s TCP/IP software is working properly. Typing in “ping localhost” should give similar results.

    The Future – IPv6

    127.x.y.z forms the special class of loopback addresses for IPv4. For IPv6, the IP address 0:0:0:0:0:0:0:1 is reserved for loopback.