Cisco® CCENT Exam Cram Notes : DNS And DHCP Servers


Go to latest CCNA Exam Cram

5. Infrastructure Services & Maintenance

1. DNS

DNS resolves Domain names with IP addresses. WINS resolves NetBIOS names to IP addresses. The core administrative unit in DNS is called a zone. A zone is a physical file composed of resource records that define a group of domains. A domain is a node in the DNS namespace and all sub-domains below it.

Any web server can be made to be present on the public Internet by using an external DNS name and a public IP address. Of course, you need to ensure physical connectivity to the Internet, and a broadband router with ISP connection is a good solution for this. Web server listens to port 80, therefore, you need to enable port 80 (incoming) on the router.

When a Cisco router is initially configured, the router's DNS lookup function is enabled by default. This function is only useful if the router utilizes a DNS server on the network otherwise, it will cause delays to users. When an erroneous URL is typed, the DNS lookup function will attempt to find the URL on the DNS server. If no DNS server is available, the user's computer will hang while the lookup is performed. To decrease user delays if no DNS server is configured, disable the DNS lookup function on the Cisco router.

2. DHCP (Dynamic Host Configuration Protocol)

DHCP is a successor to BOOT/P protocol. DHCP allows us to dynamically assign IP addresses, subnet mask,default gateway to hosts on a network. In other words, DHCP allows us to IP configure the client computers on a network dynamically at boot time.

DHCP configuration commands

ip dhcp pool <poolname>

Configures a DHCP address pool on a Cisco IOS DHCP Server and enters DHCP pool configuration mode


Provides the IP address range for allocation to the dhcp client computers.


Assigns the domain name for the dhcp client. The domain name is used as a suffix for any domain requests sent out.


Specifies the IP address of a DNS server. One IP address is required, although you can specify up to eight addresses in one command line


Specifies the IP address of a router. One IP address is required, although you can specify up to eight addresses in one command line.

Lease 2

To configure the duration of the lease for an IP address that is assigned from a Cisco IOS DHCP Server to a DHCP client, use the lease DHCP pool configuration command. To restore the default value, use the no form of this command. The default lease period is 1 day.

clear ip dhcp conflict * - Clears all ip address conflicts.

clear ip dhcp server statistics - DHCP server counters will be initialized, or set to zero, with the clear ip dhcp server statistics command.

clear ip dhcp binding * - Clears all automatic bindings.

clear ip dhcp binding a.b.c.d - Deletes the specified address binding from DHCP server database

Configure Cisco router as DHCP Client:

This is done using a single command,

Router(config-if)#ip address dhcp

Un-Configure Cisco IOS router as DHCP client:

Router(config-if)#no ip address dhcp

Note the distinction between dhcp server and dhcp client. A router may be configured in both ways. When configured as dhcp server, it provides dhcp information to the dhcp clients on the network, whereas, if configured as dhcp client, it takes dhcp information from other dhcp server on the network

show ip dhcp conflict: This command shows information about IP conflicts that are detected during the DHCP negotiation. IP conflicts occur when hosts have statically assigned IP addresses that are within the DHCP configured range, but are not excluded.

show ip dhcp binding [ip-address] : The following examples show the DHCP binding address parameters, including an IP address, an associated MAC address, a lease expiration date, and the type of address assignment that have occurred.

Example: Router>show ip dhcp binding

IP address Hardware address Lease expiration Type 00a0.9802.32fc Feb 21 2016 12:00 AM Automatic

To display Cisco IOS DHCP Server database agent information, use the "show ip dhcp database" privileged EXEC command.

show ip dhcp database [url]

Related command: ip dhcp database

Configures a Cisco IOS DHCP Server to save automatic bindings on a remote host called a database agent.

To configure your router to do look-up for domain name server, the correct syntax is:

ip name-server <ipaddress-of-server>

Here the ipaddress-of-server is the ip address of the DNS server

Service Set Identifier (SSID) is the term to identify a WLAN. In most cases SSID is broadcast by the AP, the user only needs to select that SSID and provides a correct password to access it. But in some cases for security reason, the SSID can be disabled. Users can only access to that network if they type both SSID and password correctly.

Configure the DHCP relay agent to forward packets to a DHCP server.

DHCP clients need to use User Datagram Protocol (UDP) broadcasts to send their initial DHCPDISCOVER messages because they don't have information about the network to which they are attached. If the client is on a network segment that does not include a server, UDP broadcasts normally are not forwarded because most routers are configured to not forward broadcast traffic.

You can remedy this situation by configuring the interface of your router that is receiving the broadcasts to forward certain classes of broadcasts to a helper address. You can use more than one helper address per interface.

When a router forwards these address assignment/parameter requests, it is acting as a DHCP relay agent. The Cisco router implementation of the DHCP relay agent is provided via the ip helper-address interface configuration command.

Unlike PCs or servers, Cisco network devices specifically need to run NTP to synchronize the time and date. That's because most Cisco devices don't have an internal clock. An NTP client synchronizes the time and date with an NTP server. The NTP server should be a reliable source.

To configure NTP on your IOS router, follow the steps given below:

1. Choose the NTP server your Cisco router/switches will use.

2. Find out the IP address for this server. It could be an external source such as NIST or internal.

3. Enter the following commands on the IOS device:

Router#configure terminal
Router(config)#ntp server <IP address of NTP Server>

4. Verify the association with the server using the show ntp status and show ntp associations commands.

Runts are packets that are smaller than the medium's minimum packet size. For example, Ethernet has a minimum allowed packet size of 64 bytes. Any packet that is less than 64 bytes in size is considered a runt in Ethernet.

Giants are packets that are bigger than the medium's maximum packet size. For example, Ethernet has a maximum allowed packet size of 1,518 bytes. Any packet that is bigger than 1,518 bytes is considered a Giant in Ethernet.

CRC error occurs when the check sum calculated at the receiving end of the frame does not match with the check sum calculated at the source end. The most probable reasons for runts, giants, and CRC errors is frame collisions while traveling from source to destination. It is also possible that a network card or device is bad and generating runts and giants.

Previous   Contents   Next

CCENT Cram Notes Contents
certexams ad

simulationexams ad