Ethernet Hub - Physical Layer Device

Hub is a network device connects multiple devices and makes single network segment. Hub works in Physical layer of OSI Reference Model. Since it is a layer one device, it does not have the feature to read the signals it transmit. All the devices connected to the hub feel they are connected to the same physical wire. So we can say hub has single collision domain. In other words a hub simply repeat the signal it receives. Let us check the working of hubs.

How Hubs work

When a computer connected to a hub sends packets, hub broadcast the packets to all the ports except the one it came from. Here we can see the difference between Switch and hub. Hub does not read the packets and it broadcast to all ports but Switch is intelligent device and read the packet and just unicast it. So Switch is more efficient compared to hub. Another problem with hub is the higher chances of packet collisions. We know hub has single collision domain and the chances of collision is very high.


Now a days very rarely we use hubs to connect network devices. Most networks use Switches to connect multiple network devices in a single network.

Other Tutorials

1. Router Cascading Guide
   
   
2. Router and Major Routing Protocols
   
   
3. How to Power Cycle a Router

Network Portal

Internet Control Message Protocol (ICMP)

One of the important part we need to do while troubleshooting a network is to check the connectivity of two devices. We have many utilities to check the connectivity and availability of destination device. All these utilities are based on ICMP, defined in RFC 792. Internet Control Message Protocol (ICMP) is used to send error and control information between devices which support TCP/IP. Utilities like ping, tracert, pathping etc works using ICMP. This tutorial explains the functions of Internet Control Message Protocol and the list of error messages generated by ICMP.

Important Functions of ICMP

1. Send echo requests from a TCP/IP device to another
   
   
2. Responds to Echo request
   
   
3. Helps to find routing path issues
   
   
4. ICMP echo message verify the availability of destination

List of Messages Generated by ICMP

1. Address Request
   
   
2. Address reply
   
   
3. Destination unreachable
   
   To learn more about the Destination unreachable message generated by Internet Control Message Protocol, visit the links below.
   
   
   • Destination Net Unreachable
     
     
   • Destination Host Unreachable
     
   
   
4. Subnet Mask Request
   
   
5. Time Exceeded

Network Portal

DMZ In Network Security (Demilitarized Zone)

Now most of the modern Small Office Home Office routers offer DMZ host feature. For those who don't know about the use of DMZ, this tutorial may give an overall idea about it. If you place a device in your network in DMZ, that device won't have much access to other devices connected to the network. Device placed in DMZ is more exposed to external threats as it is logically placed outside the network with limited security. Also all ports of DMZ host are open and exposed to external network. DMZ host has its on merits and demerits.

What is The Use of DMZ in a Network

If you are a hardcore online gamer, you know the hurdles created by the router firewall and NAT. In most cases simple port forwarding allows you to continue online gaming but rarely some users face issues. Ultimate solution for the firewall issues in online gaming is to place your gaming computer in DMZ. However, after this step, your gaming computer is no longer protected by router firewall. Also your computer face limited connection to other computers in your home network.

Technically DMZ host is outside the network. So other hosts in your network is safe even if DMZ host is compromised. Though DMZ computer is compromised by a hacker, attacker will not get access to other devices in the network. So you can run a web service on your computer which can be placed in DMZ without risking the security of other devices in your network.

Further Readings

1. Denial of Service (DOS) attacks Tutorial
   
   
2. How to Improve The Speed Of Your Internet Connection
   
   
3. Port Forwarding On Belkin Wireless Router
   
   
4. Port Forwarding On Linksys Wireless Router

CoreNetworkZ -Network Portal

Network Address Translation( NAT)

Network Address Translation (NAT) allows you to convert one IP address in to another address. Address Translation process changes the IP address information in a packet to different address. NAT feature is introduced to solve two major issues faced in networking: face the shortage of IP addresses and hide the network scheme. Now the major function of Address Translation is to convert the private IP addresses in a network in to a public IP address. This tutorial explains the major functions, different types and working of NAT.

Different Types of Address Translation

Address Translation is of different types depends on the way and the number of IP addresses they translate.

1. Static NAT
   
   In Static Network Address Translation, one IP address in a packet is translated to different IP address manually. It is one to one mapping.
   
   
2. Dynamic NAT
   
   In Dynamic Network Address Translation, address translation device picks an IP address dynamically to convert from a list of allowed IP addresses. Dynamic NAT has two set of IP addresses. One is a list of IP addresses which are allowed to be converted and the second is the list of IP addresses which are to be converted. The major problem with static and dynamic NAT is, they convert just one IP address  to another IP address. NAT cannot convert multiple IP addresses to single public IP address or vice versa.
   
   
3. Port Address Translation
   
   Port Address Translation (PAT) helps all the private IP addresses in the network to use single public IP address with different port numbers. So by using PAT we overcome the limitation of NAT which uses one to one mapping. One disadvantage of PAT is, not all applications work with it. PAT works with only TCP and UDP protocols.
   
   
4. Port Address Redirection
   
   Port Address Redirection (PAR) is the reverse of PAT. In PAR, multiple public IP addresses will be converted to single private IP address. PAR too works with only TCP and UDP protocols.
   

Major Benefits of Network Address Translation

1. Address Translation fixed the scarcity of IP addresses
   
   
2. NAT successfully hides internal network design from external parties
   
   
3. NAT helps network admins to have tighter control over network traffic

Disadvantages of Address Translation

Though NAT is very helpful, it is not free from disadvantages. Some of them are:

1. PAT and PAR works with only TCP and UDP protocols
   
   
2. Not all applications support Address Translation
   
   
3. Address Translation cause extra delay in each connection
   
   
4. NAT makes network troubleshooting more complex

Recommended Tutorials

1. Dynamic Host Configuration Protocol (DHCP)
   
   
2. OSI Reference Model
   
   
3. Virtual Router Redundancy Protocol (VRRP)
   
   
4. What is a Router & Major Routing Protocols

CoreNetworkZ - Network Portal

Dynamic Host Configuration Protocol (DHCP) CCNA Exam Tutorial

The Dynamic Host Configuration Protocol (DHCP) allows devices to dynamically get IP address information. It is very important in networks especially for big networks because it assigns IP addresses to each devices in the network automatically. Think about a network having thousands of devices. It is very difficult to assign static IP addresses to each devices without having errors. Here Dynamic Host Configuration Protocol helps us. This protocol assigns IP address to each device without any errors and is very easy to manage. It contains two parts, server and client.

Working of Dynamic Host Configuration Protocol

We can simply use the term DORA to explain the working of DHCP. It is a four stage process and each stage is explained below.

1. D-- DHCP Discovery
   
   When a client connected to a network, it sends broadcast to discover the DHCP server. This broadcast is termed as DHCPDISCOVER.
   
   
2. O--- DHCP Offer
   
   At this stage, server responds to the client DISCOVERY message and sends a DHCPOFFER unicast message. DHCPOFFER includes IP address, subnet mask, default gateway address, DNS address etc.
   
   
3. R---- DHCP Request
   
   Here client sends a DHCPREQUEST message to the server.
   
   
4. A--- DHCP Acknowledgment
   
   Here server sends an acknowledgment (DHCPACK) to the client indicating it received the DHCPREQUEST from the client and client accepted the IP address information.

Advantages of Using DHCP

Major advantages of using Dynamic Host Configuration Protocol are:

1. Very less chances to get IP addressing errors
   
   
2. Efficient control on IP addressing in a network because it is centralized.
   
   
3. We can avoid the heavy work of manual IP addressing

Suggested Topics

1. How to Configure a Computer to Receive Automatic IP Address
   
   
2. Interior Gateway Routing Protocol (IGRP)
   
   
3. Internet Group Management Protocol (IGMP) Tutorial
   
   
4. Working Of Remote Procedure Call (RPC)in Session Layer

Cisco Certification Exam Tips

If you are preparing for Cisco certification exams like CCNA, you must have good knowledge in DHCP. If you follow CCNA dump for your next exam, the dotted information might help you. Some important points to remember about Dynamic Host Configuration Protocol for your Cisco Certification exams are listed below.

• Command to Enable or Disable DHCP on Cisco IOS Router --service dhcp
  
  
• Cisco Command to create address pool -- ip dhcp pool
  
  
• Command to assign domain name to client --domain-name
  
  
• Command to specify DHCP IP range -- network
  
  
• Command to check if server can use an IP address in the pool -ip dhcp ping timeout
  
  
• Command to exclude an IP address from the DHCP address pool --ip dhcp excluded-address
  
  
• Show addresses assigned to clients --show ip dhcp binding
  
  
• DHCPNACK - This acknowledgement from server tells the client the offer is no longer valid
  
  
• DHCPDECLINE - Client tells the server, it cannot use the offer from the server

Network Portal

Fundamentals of Ethernet

Ethernet is LAN media type that uses CSMA/CD to send information and functions at Data Link Layer of OSI Reference Model. Ethernet media type having different physical topologies but having single logical topology: bus topology. With various physical topologies we can design an this network with different shapes but the communication happens in Bus topology only. It has two versions right now. One of them is DIX version and the second is IEEE version. This tutorial explains various topologies and versions of Ethernet.

Physical Topologies Supported

1. Bus topology
   
   
2. Star Topology
   
   
3. Point to Point

Two Versions Of Ethernet

1. DIX Version
   
   DIX  Ethernet or Ethernet 2 is widely using now. This version does not split Data Link layer in to two. Ethernet 2 has frame value more than 1500. This standard is designed by the combined work of three companies. Those companies are DEC, Intel and Xerox. This standard is named as DIX by taking the first letter from the names of these three companies.
   
   
2. IEEE Version
   
   This version of Ethernet is defined under IEEE 802.2/3. Here data link layer is sub divided in to LLC and MAC. To read more about it, visit the links below.
   
   
   • LLC (Logical Link Control) IEEE 802.2
     
     
   • MAC Sublayer Defined in IEEE 802.3
   
   Frame value of IEEE802.3 is less than 1500. This is the major difference between IEEE and DIX versions.
   

To read more about the devices and protocols work in standard, visit the link below.
Data Link Layer in OSI Reference Model

Technology Portal

What is WiFi & Different IEEE 802.11 Wireless Standards

Wi-Fi is the term used to denote Wireless Local Area Networks which are using IEEE802.11 wireless standards. Now the term WiFi is representing all Wireless Local Area Networks. Wireless technologies used in WiFi are IEEE 802.11a, b, g, n and ac. In this tutorial let us check what is WiFi and the major 802.11 standards. You can also see a list of WiFi devices which supports them. At the end of the tutorial, you can see a backward compatibility checker chart. Using this chart you can take decisions related to extending your WLAN networks. With the help of the chart you can avoid situations of buying a device which is not compatible with existing devices (standards which are not backward compatible).

IEEE 802.11 Wireless Standards Available In Industry

1. IEEE802.11a
   
   
2. IEEE802.11b
   
   
3. IEEE802.11g
   
   
4. IEEE802.11n
   
   
5. IEEE802.11ac

Compatibility Between Different 802.11 Wireless Standards

Not all 802.11 technologies are backward compatible because of the different frequencies supported by different standards. So if you are planning to extend your current WLAN with new devices, you must make sure the new wireless devices you are going to buy are backward compatible with existing devices. So it is important to know the technologies supported by the existing equipments. Once you have noted down the technologies that are currently implemented, you may check which all 802.11 standards are compatible with them. You can see below a list of different WiFi standards which support each other. If you see 'Yes' in that chart, it shows the technologies those in the row and column are backward compatible. On the other hand if you see 'No', it tells the standards listed ion row and column are not backward compatible.

WiFi Standard	IEEE802.11a	IEEE802.11b	IEEE802.11g	IEEE802.11n	IEEE802.11ac
11a	Yes	No	No	Yes	Yes
11b	No	Yes	Yes	Yes	No
11g	No	Yes	Yes	Yes	No
11n	Yes	Yes	Yes	Yes	Yes
11ac	Yes	No	No	Yes	Yes

Frequencies Supported

Frequency mode supported is the reason for the incompatibility between these IEEE technologies.

1. 11a ---5 GHz

2. 11b ---2.4 GHz

3. 11g ---2.4 GHz

4. 11n -- Supports both 2.4GHz and 5GHz

5. 11ac -- Supports 5GHz

List Of 802.11 Wireless Devices

1. Belkin F5D8233-4  N Router Supports IEEE802.11n
   
   
2. D-Link  Router Which Supports Various WiFi Standards
   
   
3. D-Link  Access Point DWL-G700AP
   
   
4. Configure Adapter in AdHoc mode

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