Briefly describe and compare the five basic network topologies. Describe the two

Question

Briefly describe and compare the five basic network topologies.

Describe the two parts of an IP address and the role of the network mask.

Provide a step-by-step description of how a packet is routed, assuming that it must traverse at least one router.

What is a socket? How does it relate to a typical networking API?

Explain the role of port numbers in establishing a connection between two processes.

Explain the role of ARP and how it works.

What is the difference between a global IP address and a private IP address?

Explanation / Answer

Network Topology refers to layout of a network and how different nodes in a network are connected to each other and how they communicate. Topologies are either physical (the physical layout of devices on a network) or logical (the way that the signals act on the network media, or the way that the data passes through the network from one device to the next)

The 5 most important and commonly used network topologies

1- Bus Topology

In Bus topology, all nodes (or devices) are connected to a common communication medium. Usually, a central cable is used as communication medium. This cable is called Bus. The computers or nodes are connected to theBus through interface connector. The ends of bus (central cable) are closed with terminators. These terminators are used to absorb signals.
In bus network, each computer is assigned a unique address. When a computer sends a message to another computer, it also attaches the address of the destination computer with message. This message moves from one computer to other through the bus. Each computer connected to the network checks the message. If the address (attached with message) matches with the address of computer, then it accepts the message. Otherwise, the message moves to the next computer. In this way, the message is received by the destination computer.

Advantages

Disadvantages

2- Star Topology

    In a star network, all computers or nodes are directly connected to a central device. The central device is called Hub. Typically, the nodes are connected to the Hub with unshielded twisted pair (UTP). Star topology is most commonly used in LAN. This form of network shape looks like a Star.
In Star topology, the computers communicate with each other through central Hub. The sending computer sends data to the Hub. The Hub sends data to the receiving computer. In this way, data is transferred from one computer to another.

Advantages

Disadvantages

3- Ring Topology
In ring topology, each computer is connected to the next computer and the last computer is connected to the first. Thus, a ring of computers is formed.
When a computer sends a message to another computer on the network, the message flows from one computer to the next computer. Every computer receives message from previous computer and transmits it to the next computer until the destination computer receives the message. Token passing method is used in this topology.

Advantages

Disadvantages

4- Tree Topology
    Tree topology has combined features of bus and star topology. Typically to form a tree network, multiple star topologies are combined together through central cable or bus. The tree network looks like a tree structure.
In tree topology network, one central hub and multiple secondary hubs are used. The central hub is an active hub. It contains a repeater (a hardware device), which regenerates the received signals of data. The secondary hubs usually are passive hubs. Each passive hub controls the nodes directly connected to it. It also exchanges data to other devices connected to the other secondary hubs (or same hub) through the central hub. The secondary hub may also act as active hub if another secondary hub is directly connected to it. The cable TV network is an example of tree topology. In this network, main cable is divided into branches and each branch is further divided into smaller branches and so on. The hub is used when a new branch is created.

Advantages
The tree topology has the same advantages as star topology but it has some additional advantages. The main advantage is:

Disadvantages
      The tree topology also has the same disadvantages as star topology but it has some additional disadvantages. The main disadvantage is:

5- Mesh Topology
    In mesh topology, each node is directly connected to every other node on the network. This type of network involves the concept of routes. Mesh topology is rarely used in LAN. Some WANs like the Internet use mesh topology.
In mesh topology, each node can send message to the destination through multiple paths. It means that each node of mesh network has several possible paths to send or to receive message.

Advantages

Disadvantages

Describe the two parts of an IP address and the role of the network mask

An IP address consists of two parts, one identifying the network and one identifying the node, or host.

The Class of the address determines which part belongs to the network address and which part belongs to the node address. All nodes on a given network share the same network prefix but must have a unique host number.

Class A Network

In a Class A Network binary address start with 0, therefore the decimal number can be anywhere from 1 to 126. The first 8 bits (the first octet) identify the network and the remaining 24 bits indicate the host within the network. An example of a Class A IP address is 102.168.212.226, where "102" identifies the network and "168.212.226" identifies the host on that network.

Class B Network

In a Class B Network, binary addresses start with 10, therefore the decimal number can be anywhere from 128 to 191. The number 127 is reserved for loopback and is used for internal testing on the local machine. The first 16 bits (the first two octets) identify the network and the remaining 16 bits indicate the host within the network. An example of a Class B IP address is 168.212.226.204 where "168.212" identifies the network and "226.204" identifies the host on that network.

Class C Network

Binary addresses start with 110, therefore the decimal number can be anywhere from 192 to 223. The first 24 bits (the first three octets) identify the network and the remaining 8 bits indicate the host within the network. An example of a Class C IP address is 200.168.212.226 where "200.168.212" identifies the network and "226" identifies the host on that network.

Class D Network

In a Class D Network, binary addresses start with 1110, therefore the decimal number can be anywhere from 224 to 239. Class D networks are used to support multicasting.

Class E Network

In a Class E Network, binary addresses start with 1111, therefore the decimal number can be anywhere from 240 to 255. Class E networks are used for experimentation. They have never been documented or utilized in a standard way.

A Subnet mask is a 32-bit number that masks an IP address, and divides the IP address into network address and host address. Subnet Mask is made by setting network bits to all "1"s and setting host bits to all "0"s. Within a given network, two host addresses are reserved for special purpose, and cannot be assigned to hosts. The "0" address is assigned a network address and "255" is assigned to a broadcast address, and they cannot be assigned to hosts.

THE IP ROUTING PROCESS

1) Host A opens a command prompt and enters >Ping 200.200.200.5.

2) IP works with the Address Resolution Protocol (ARP) to determine which network this packet is destined for by looking at the IP address and the subnet mask of Host A. Since this is a request for a remote host, which means it is not destined to be sent to a host on the local network, the packet must be sent to the router (the gateway for Network A) so that it can be routed to the correct remote network (which is Network B).

3) Now, for Host A to send the packet to the router, it needs to know the hardware address of the router's interface which is connected to its network (Network A), in case you didn't realise, we are talking about the MAC (Media Access Control) address of interface E0. To get the hardware address, Host A looks in its ARP cache - a memory location where these MAC addresses are stored for a few seconds

4) If it doesn't find it in there it means that either a long time has passed since it last contacted the router or it simply hasn't resolved the IP address of the router (192.168.0.1) to a hardware address (MAC). So it then sends an ARP broadcast. This broadcast contains the following "What is the hardware (MAC) address for IP 192.168.0.1 ? ". The router identifies that IP address as its own and must answer, so it sends back to Host A a reply, giving it the MAC address of its E0 interface. This is also one of the reasons why sometimes the first "ping" will timeout. Because it takes some time for an ARP to be sent and the requested machine to respond with its MAC address, by the time all that happens, the TTL (Time To Live) of the first ping packet has expired, so it times out!

5) The router responds with the hardware address of its E0 interface, to which the 192.168.0.1 IP is bound. Host A now has everything it needs in order to transmit a packet out on the local network to the router. Now, the Network Layer hands down to the Datalink Layer the packet it generated with the ping (ICMP echo request), along with the hardware address of the router. This packet includes the source and destination IP address as well as the ICMP echo request which was specified in the Network Layer.

6) The Datalink Layer of Host A creates a frame, which encapsulates the packet with the information needed to transmit on the local network. This includes the source and destination hardware address (MAC) and the type field which specifies the Network Layer protocol e.g IPv4 (that's the IP version we use), ARP. At the end of the frame, in the FCS portion of the frame, the Datalink Layer will stick a Cyclic Redundancy Check (CRC) to make sure the receiving machine (the router) can figure out if the frame it received has been corrupted. 7) The Datalink Layer of Host A hands the frame to the Physical layer which encodes the 1s and 0s into a digital signal and transmits this out on the local physical network.

8)The signal is picked up by the router's E0 interface and reads the frame. It will first do a CRC check and compare it with the CRC value Host A added to this frame, to make sure the frame is not corrupt.

9)After that, the destination hardware address (MAC) of the received frame is checked. Since this will be a match, the type field in the frame will be checked to see what the router should do with the data packet. IP is in the type field, and the router hands the packet to the IP protocol running on the router. The frame is stripped and the original packet that was generated by Host A is now in the router's buffer.

10) IP looks at the packet's destination IP address to determine if the packet is for the router. Since the destination IP address is 200.200.200.5, the router determines from the routing table that 200.200.200.0 is a directly connected network on interface E1.

11) The router places the packet in the buffer of interface E1. The router needs to create a frame to send the packet to the destination host. First, the router looks in the ARP cache to determine whether the hardware address has already been resolved from a prior communication. If it is not in the ARP cache, the router sends an ARP broadcast out E1 to find the hardware address of 200.200.200.5

12) Host B responds with the hardware address of its network interface card with an ARP reply. The router's E1 interface now has everything it needs to send the packet to the final destination.

13)The frame generated from the router's E1 interface has the source hardware address of E1 interface and the hardware destination address of Host B's network interface card. However, the most important thing here is that even though the frame's source and destination hardware address changed at every interface of the router it was sent to and from, the IP source and destination addresses never changed. The packet was never modified at all, only the frame changed.

14) Host B receives the frame and runs a CRC. If that checks out, it discards the frame and hands the packet to IP. IP will then check the destination IP address. Since the IP destination address matches the IP configuration of Host B, it looks in the protocol field of the packet to determine the purpose of the packet.

15) Since the packet is an ICMP echo request, Host B generates a new ICMP echo-reply packet with a source IP address of Host B and a destination IP address of Host A. The process starts all over again, except that it goes in the opposite direction. However, the hardware address of each device along the path is already known, so each device only needs to look in its ARP cache to determine the hardware (MAC) address of each interface.

Socket

A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to. An endpoint is a combination of an IP address and a port number.

difference between a global IP address and a private IP address

A globalc IP address is the address that is assigned to a computing device to allow direct access over the Internet. A web server, email server and any server device directly accessible from the Internet are candidate for a globalc IP address. A globalc IP address is globally unique, and can only be assigned to an unique device. A private IP address is the address space allocated by InterNIC to allow organizations to create their own private network. There are three IP blocks (1 class A, 1 class B and 1 class C) reserved for a private use. The computers, tablets and smartphones sitting behind your home, and the personal computers within an organizations are usually assigned private IP addresses. A network printer residing in your home is assigned a private address so that only your family can print to your local printer.

When a computer is assigned a private IP address, the local devices sees this computer via it's private IP address. However, the devices residing outside of your local network cannot directly communicate via the private IP address, but uses your router's globalc IP address to communicate. To allow direct access to a local device which is assigned a private IP address, a Network Address Translator (NAT) should be used

Explain the role of ARP and how it works.

ARP stands for Address Resolution Protocol. It is used to associate a layer 3 (Network layer) address (such as an IP address) with a layer 2 (Data Link layer) address (MAC address).

ARP OPERATION FOR A LOCAL HOST

Your computer will have data that it needs to send (I'm assuming that we're using TCP/IP from here on). When the data gets to the Network layer it will put on the destination IP address. All of this info (the network layer datagram, aka packet) is passed down to the data link layer where it is taken and placed within a data link frame. Based on the IP address (and the subnet mask), your computer should be able to figure out if the destination IP is a local IP or not. If the IP is local, your computer will look in it's ARP table (a table where the responses to previous ARP requests are cached) to find the MAC address. If it's not there, then your computer will broadcast an ARP request to find out the MAC address for the destination IP. Since this request is broadcast, all machines on the LAN will receive it and examine the contents. If the IP address in the request is their own, they'll reply. On receiving this information, your computer will update it's ARP table to include the new information and will then send out the frame (addressed with the destination host's MAC address).

ARP OPERATION FOR A REMOTE HOST

If the IP is not local then the gateway (router) will see this (remember, the ARP request is broadcast so all hosts on the LAN will see the request). The router will look in it's routing table and if it has a route to the destination network, then it will reply with it's <i>own</i> MAC address.

This is only the case if your own computer doesn't know anything about the network topology. In most cases, your computer knows the subnet mask and has a default gateway set. Because of this, your own computer can figure out for itself that the packet is not destined for the local network. Instead, your computer will use the MAC address of the default gateway (which it will either have in it's ARP table or have to send out an ARP request for as outlined above). When the default gateway (router) receives the frame it will see that the MAC address matches it's own, so the frame must be for it. The router will un-encapsulate the data link frame and pass the data part up to the network layer. At the network layer, the router will see that the destination IP address (contained in the header of the IP packet) does not match it's own (remember, the IP address has not been touched at all in this process since your computer created the IP packet). The router will realise that this is a packet that is supposed to be routed. The router will look in it's routing table for the closest match to the destination IP in order to figure out which interface to send the packet out on. When a match is found, the router will create a new data link frame addressed to the next hop (and if the router doesn't know the hardware address for the next hop it will request it using the appropriate means for the technology in question). The data portion of this frame will contain the complete IP packet (where the destination IP address remains unchanged) and is sent out the appropriate interface. This process will continue at each router along the way until the information reaches a router connected to the destination network. It will see that the packet is addressed to a host that's on a directly connected network (the closest match you can get for an address, short of the packet being addressed to you). It will send out an ARP request for MAC address of the destination IP (assuming it doesn't already have it in it's table) and then address it to the destination's MAC address. Now then, I did slightly gloss over 1 part in the above explanation and that's the part about the router finding out the hardware address for the next hop. I just didn't want to disturb the flow with entering into that there. How the router does this will depend on what type of connection (and in some cases, what protocol and/or encapsulation is used on the connection). In some cases, this will be a hard set value (like a frame relay pvc) within the configuration of the router. In some cases, you don't even need a hardware address (like any point to point connection, there's only 1 possible host you could send it to), in those cases the router will just create a data link frame appropriate for the connection and it won't even need to be addressed. This is why the OSI model is good. It's layered so that any layer can change and as long as it takes in information in a standard way (the way the layer above wants to send it) and spits out information in a standard way (the way the layer below wants to receive it), then it's all good. When Frame Relay came along nothing changed with the way you had to address IP packets, all of the changes took place at the data link and physical layers. If some new type of connection comes along in the future, only the data link and physical layers will likely change. When we go to IPv6, only the network layer should

the role of port numbers in establishing a connection between two processes

a port is an endpoint of communication in an operating system. While the term is also used for hardware devices, in software it is a logical construct that identifies a specific process or a type of service.

A port is always associated with an IP address of a host and the protocol type of the communication, and thus completes the destination or origination address of a communications session. A port is identified for each address and protocol by a 16-bit number, commonly known as the port number.

Specific port numbers are often used to identify specific services. Of the thousands of enumerated ports, 1024 well-known port numbers are reserved by convention to identify specific service types on a host. The protocols that primarily use ports are the transport layer protocols, such as the Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) of the Internet protocol suite.

Ports are unnecessary on direct point-to-point links when the computers at each end can only run one program at a time. Ports became necessary after computers became capable of executing more than one program at a time and were connected to modern packet-switched networks. In the client–server model of application architecture, the ports that network clients connect to for service initiation provide a multiplexing service. After initial communication binds to the well-known port number, this port is freed by switching each instance of service requests to a dedicated, connection-specific port number, so that additional clients can be serviced

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