What IP address is used as a loopback address and is not a valid IP address that can be assigned to a network?

Try the new Google Books

Check out the new look and enjoy easier access to your favorite features

Try the new Google Books

Check out the new look and enjoy easier access to your favorite features

There are a few reserved IPv4 address spaces which cannot be used on the internet. These addresses serve special purpose and cannot be routed outside the Local Area Network.

Private IP Addresses

Every class of IP, (A, B & C) has some addresses reserved as Private IP addresses. These IPs can be used within a network, campus, company and are private to it. These addresses cannot be routed on the Internet, so packets containing these private addresses are dropped by the Routers.

In order to communicate with the outside world, these IP addresses must have to be translated to some public IP addresses using NAT process, or Web Proxy server can be used.

The sole purpose to create a separate range of private addresses is to control assignment of already-limited IPv4 address pool. By using a private address range within LAN, the requirement of IPv4 addresses has globally decreased significantly. It has also helped delaying the IPv4 address exhaustion.

IP class, while using private address range, can be chosen as per the size and requirement of the organization. Larger organizations may choose class A private IP address range where smaller organizations may opt for class C. These IP addresses can be further sub-netted and assigned to departments within an organization.

Loopback IP Addresses

The IP address range 127.0.0.0 – 127.255.255.255 is reserved for loopback, i.e. a Host’s self-address, also known as localhost address. This loopback IP address is managed entirely by and within the operating system. Loopback addresses, enable the Server and Client processes on a single system to communicate with each other. When a process creates a packet with destination address as loopback address, the operating system loops it back to itself without having any interference of NIC.

Data sent on loopback is forwarded by the operating system to a virtual network interface within operating system. This address is mostly used for testing purposes like client-server architecture on a single machine. Other than that, if a host machine can successfully ping 127.0.0.1 or any IP from loopback range, implies that the TCP/IP software stack on the machine is successfully loaded and working.

Link-local Addresses

In case a host is not able to acquire an IP address from the DHCP server and it has not been assigned any IP address manually, the host can assign itself an IP address from a range of reserved Link-local addresses. Link local address ranges from 169.254.0.0 -- 169.254.255.255.

Assume a network segment where all systems are configured to acquire IP addresses from a DHCP server connected to the same network segment. If the DHCP server is not available, no host on the segment will be able to communicate to any other. Windows (98 or later), and Mac OS (8.0 or later) supports this functionality of self-configuration of Link-local IP address. In absence of DHCP server, every host machine randomly chooses an IP address from the above mentioned range and then checks to ascertain by means of ARP, if some other host also has not configured itself with the same IP address. Once all hosts are using link local addresses of same range, they can communicate with each other.

These IP addresses cannot help system to communicate when they do not belong to the same physical or logical segment. These IPs are also not routable.

Question 1

Which IP address can be assigned to an Internet interface?

A. 10.180.48.224 B. 9.255.255.10 C. 192.168.20.223 

D. 172.16.200.18

Answer: B

Explanation

The IP address which can be assigned to an Internet interface is a public IP address. Private IP address are found in the following ranges:

* From 10.0.0.0 to 10.255.255.255* From 172.16.0.0 to 172.31.255.255

* From 192.168.0.0 to 192.168.255.255

Also some special IP addresses (like the local loopback address 127.0.0.1, multicast addresses…) can’t be assigned to an Internet interface.

In this question only answer B doesn’t belong to the range of private IP address -> B is the correct answer.

Question 2

What will happen if a private IP address is assigned to a public interface connected to an ISP?

A. Addresses in a private range will be not routed on the Internet backbone.B. Only the ISP router will have the capability to access the public network.C. The NAT process will be used to translate this address in a valid IP address.D. Several automated methods will be necessary on the private network.

E. A conflict of IP addresses happens, because other public routers can use the same range.

Answer: A

Question 3

When is it necessary to use a public IP address on a routing interface?

A. Connect a router on a local network.B. Connect a router to another router.C. Allow distribution of routes between networks.D. Translate a private IP address.

E. Connect a network to the Internet.

Answer: E

Question 4

When a DHCP server is configured, which two IP addresses should never be assignable to hosts? (Choose two)

A. network or subnetwork IP address B. broadcast address on the networkC. IP address leased to the LAND. IP address used by the interfacesE. manually assigned address to the clients 

F. designated IP address to the DHCP server

Answer: A B

Explanation

Network or subnetwork IP address (for example 11.0.0.0/8 or 13.1.0.0/16) and broadcast address (for example 23.2.1.255/24) should never be assignable to hosts. When try to assign these addresses to hosts, you will receive an error message saying that they can’t be assignable.

Question 6

The network administrator has asked you to check the status of the workstation’s IP stack by pinging the loopback address. Which address would you ping to perform this task?

A. 10.1.1.1 B. 127.0.0.1 C. 192.168.0.1 

D. 239.1.1.1

Answer: B

Explanation

The IP address of 127.0.0.1 is the well-known loopback IP address on a computer. When try pinging this address, you are testing if the TCP/IP stack is working or not.

Question 7

Which command is used to see the path taken by packets across an IP network?

A. show ip routeB. show routeC. traceroute

D. trace ip route

Answer: C

Explanation

Notice that “traceroute” is the command used on a Cisco router while the “tracert” is the command used in MSDOS of Windows. They have the same purpose of finding the path that packets use to reach a destination network.

Question 8

Refer to the exhibit. Which value will be configured for Default Gateway of the Local Area Connection?

A. 10.0.0.0B. 10.0.0.254C. 192.223.129.0

D. 192.223.129.254

Answer: B

Explanation

The default gateway IP address must be on the same network with the configured host’s IP address and not is the network or broadcast address -> B is correct.

Question 9

Which of the following describe private IP addresses? (Choose two)

A. addresses chosen by a company to communicate with the Internet B. addresses that cannot be routed through the public InternetC. addresses that can be routed through the public InternetD. a scheme to conserve public addresses

E. addresses licensed to enterprises or ISPs by an Internet registry organization

Answer: B D

Question 10

Refer to the exhibit. A network technician is asked to design a small network with redundancy. The exhibit represents this design, with all hosts configured in the same VLAN. What conclusions can be made about this design?

A. The design will function as intendedB. Spanning-tree will need to be used.C. The router will not accept the addressing scheme.D. The connection between switches should be a trunk.

E. The router interfaces must be encapsulated with the 802.1Q protocol.

Answer: C

Explanation

Each interface on a router must be in a different network. If two interfaces are in the same network, the router will not accept it and show error when the administrator assigns it.

The Internet Protocol (IP) specifies a loopback network with the (IPv4) address 127.0.0.0/8. Most IP implementations support a loopback interface (lo0) to represent the loopback facility. Any traffic that a computer program sends on the loopback network is addressed to the same computer. The most commonly used IP address on the loopback network is 127.0.0.1 for IPv4 and ::1 for IPv6. The standard domain name for the address is localhost.

A network device also includes an internal loopback address (lo0.16384). The internal loopback address is a particular instance of the loopback address with the logical unit number 16384.

The loopback interface is used to identify the device. While any interface address can be used to determine if the device is online, the loopback address is the preferred method. Whereas interfaces might be removed or addresses changed based on network topology changes, the loopback address never changes.

When you ping an individual interface address, the results do not always indicate the health of the device. For example, a subnet mismatch in the configuration of two endpoints on a point-to-point link makes the link appear to be inoperable. Pinging the interface to determine whether the device is online provides a misleading result. An interface might be unavailable because of a problem unrelated to the device's configuration or operation. You can use the loopback interface to address these issues.

Benefits of Loopback Interface

• As the loopback address never changes, it is the best way to identify a device in the network.

• The loopback interface is always up and it is reachable as long as the route to that IP address is available in the IP routing table. Hence you can use the loopback interface for diagnostics and troubleshooting purposes.

• Protocols such as OSPF use the loopback address to determine protocol-specific properties for the device or network. Further, some commands such as ping mpls require a loopback address to function correctly.

• You can apply stateless firewall filters to the loopback address to filter packets originating from, or destined for, the Routing Engine.

• Junos OS creates the loopback interface for the internal routing instance, which prevents any filter on lo0.0 from disrupting internal traffic.