layer 3 « Class in Session: Cisco

Posts Tagged 'layer 3'

Dynamic Host Configuration Protocol (DHCP)

Published September 1, 2009 David Stahl , Routing & Switching 1 Comment
Tags: DHCP, layer 3

As presented in previous posts, there are four tools that are used 24/7 in almost every TCP/IP network in the world to help the network layer – Layer 3 – with its task of routing packets end-to-end through an internetwork. These automated tools are identified as:

Address Resolution Protocol (ARP)
Domain Name Service (DNS)
Dynamic Host Configuration Protocol (DHCP)
Ping

And, every device that uses TCP/IP, in fact every interface on every device, needs a valid IP address. For some of these devices, the IP address can, and should, be statically assigned through configuration of the individual device. Of course, the method used to assign IP addresses to interfaces is, many times, a matter of organizational policies and procedures.

As an example, all of the commonly-used computer operating systems that support TCP/IP enable the user to statically configure the IP address on each interface. For instance, routers and switches typically use statically configured IP addresses. Also, servers typically use statically configured IP addresses. Using a statically configured and seldom-changed IP address helps because all references to that server can stay the same over time.

It’s the same concept as knowing the address of your favorite electronics store. The location never changes and you always know how to get there from your home, on the way home from work, or from somewhere else. In a similar manner, if servers have a static, unchanging IP address, the users of that server know how to reach the server from anywhere, consistently.

However, the end-user device host computer does not, necessarily, need to use the same IP address every day. Thinking back to the example of your favorite electronics store, you could move to a new apartment every week, but you still know where the electronics store is. The workers at the store do not need to know where you live. Likewise, servers typically don’t care that your PC has a different IP address today than yesterday.

As computers become smaller and lighter, it is more common for them to move from one network to another, where they would require a different IP address and network and or subnetwork. Actually, we find that in the course of a normal business day, many laptops and palmtops could move from one network to another many times per day. In reality, we find that on a worldwide basis, DHCP is used for everything from assigning IP addresses for corporate networks with thousands of hosts, to enabling a home Internet access router to automatically provide the correct Internet configuration to a single user’s computer.

DHCP defines the protocols used to allow computers and other devices to request a permanent IP address or lease a temporary address. The DHCP process is modeled on the client-server configuration. The DHCP process uses a server that maintains a list of pools of IP addresses available in each subnet. The user devices, or DHCP clients, can send the DHCP server a message, asking to be permanently assigned, or to lease an IP address. The DHCP server then suggests an IP address and, if accepted by the DHCP client, the server notes that the IP address is no longer available for assignment to any other hosts and the client has an address to use.

When this dynamic DHCP IP address allocation is used, the network administrator must configure the parameters on the DHCP server to control how leases are assigned, managed and, most importantly, how long the administrator wants the client leases to last. The administrator can set the lease time to infinity or, in essence, a permanent lease. However, administrators can choose a number that best suits the network. Some of the most often-used lease periods are:

One Hour or Less
One Day
Three Days
One Week
One Month
Three Months
One Year

It’s interesting to note that with the DHCP process, clients no longer own their IP addresses. They lease them from the true owner, the DHCP server. As such, the primary job of both the DHCP server and the administrator of that server is to maintain and manage client addresses.

When properly configured, a DHCP server can also supply additional information to a client. For example, in addition to learning their IP address, a host needs to know the subnet mask to use, plus what default gateway to use, as well as the IP address(es) of any DNS servers. In most networks today, DHCP supplies all of these facts to a typical end-user host.

Author: David Stahl

Network Layer Utilities: End-to-End Data Delivery

Published August 22, 2009 David Stahl , Routing & Switching Leave a Comment
Tags: layer 2, layer 3, OSI Model

From our studies, we know that the International Organization for Standards (ISO) created the Open Systems Interconnection (OSI) networking model to standardize data networking protocols, to enable communication between all computers and devices across any network anywhere in the world. The OSI model is now mainly used as a point of reference for discussing the specifications of protocols used in network design and operation. The upper layers of the OSI reference model (application, presentation, and session = Layers 7, 6, and 5) define functions focused on the application. The lower four layers (transport, network, data link, and physical = Layers 4, 3, 2, and 1) define functions focused on end-to-end delivery of the data.

When we consider the seven layers of the OSI Reference Model, there are two that deal with addressing the data link layer and the network layer. The physical layer is not strictly concerned with addressing at all, only sending at the bit level. The layers above the network layer all work with network layer addresses.

When we discuss end-to-end delivery of data, we must necessarily talk about how datagrams are addressed. We find out that addressing is done at two different layers of the OSI model and two different layers are used, which are very different types of addresses that are used for different purposes. Layer 2 addresses, such as IEEE 802 MAC addresses, are used for local transmissions between hardware devices that can communicate directly. They are used to implement basic LAN, WLAN, and WAN technologies. In contrast, layer 3 addresses, which are most commonly 32-bit Internet Protocol addresses, are used in internetworking to create a virtual network at the network layer.

The most important difference between these types of addresses is the distinction between layers 2 and 3 themselves. Layer 2 MAC addresses enable communication between directly-connected devices residing on the same physical network. Layer 3 IP addresses allow communications between both directly and indirectly-connected devices.

For example, say you want to connect to the Web server at http://www.cisco.com. This is a Cisco Web site that resides on a server that has an Ethernet card used for connecting to its Internet service provider site. However, even if you know its Layer 2 MAC address, you cannot use it to talk directly to this server using the Ethernet card in your home PC. This is because these two devices are on different networks. In fact, they may even be on different continents!

Instead, these devices communicate at layer 3, using the Internet Protocol and higher layer protocols such as TCP and HTTP. Your request is routed from your home machine through a sequence of routers to the Cisco server. The response is then routed back to you. The communication is, logically, at layers 3 and above. You send the request, not to the MAC address of the server’s network card, but rather to the server’s IP address.

While we can virtually connect devices at Layer 3 through routers, these connections are really conceptual only. When you send a datagram that has been created using the OSI 7-Layer-Model, it is sent one hop at a time, from one router to another, from one physical network to the next. At each of these hops, an actual transmission occurs at the physical and data link layers.

When your request is sent to your local router at layer 3, which is usually referred to as your default gateway, the actual request is encapsulated in an Ethernet frame using whatever method you use to physically connect to the router. It is addressed and sent to the default gateway router using the router’s data link layer MAC address. The same happens for each subsequent step until, finally, the router nearest the Cisco Web server, sends the datagram to the destination using the data link (MAC) address of the NIC card of the Cisco Web server.

In my next blog, I will discuss the Address Resolution Protocol (ARP) that is a method used for finding a device’s link layer MAC hardware address when only its Internet Layer IP address is known.

Author: David Stahl

Network Layer Utilities: DNS

Published August 10, 2009 David Stahl , Routing & Switching 1 Comment
Tags: DNS, layer 3

Although it doesn’t always seem that way, CCDAs and CCDPs, as network designers, usually try and make any network, no matter how complex, as simple as possible to use. And, we human beings often need to automate many networking tasks to provide a basic simplicity of network operation.

There are four tools that are used 24/7 in almost every TCP/IP network in the world to help the network layer (Layer 3) with its task of routing packets end-to-end through an internetwork. These automated tools are identified as:

Address Resolution Protocol (ARP)
Domain Name Service (DNS)
Dynamic Host Configuration Protocol (DHCP)
Ping

One of these tools is the Domain Name Service (DNS). We know that both hosts and routers participate in the IP routing process. This defines how an IP packet can be delivered from the host at which the packet is created to the destination host. However, it would be very difficult for all of us to remember every IP address of every network device with which we wish to communicate, either locally in our Local Area Networks or on the World Wide Web. Human beings are just are not that good at remembering strings of numbers. We are good at remembering words, however, and that is where DNS names come in. You probably have hundreds of domain names stored in your head.

Continue reading ‘Network Layer Utilities: DNS’