Network Engineer Interview Questions

The following questions and answers are from the ebook 250+ Network Engineer Interview Questions and answers

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What is an IP Helper address feature and why is it required in a DHCP environment

DHCP Discover packets are broadcast packets. This means that a DHCP Discover packet which is sent from the client would not reach the DHCP Server, if the server resides on a different network. This is because, routers are required for communication between different networks and routers do not forward broadcast packets. The IP Helper address feature is configured on the router. The feature informs the router the DHCP Servers IP address for the network. So, when the router receives the DHCP Discover packet, it would convert it from broadcast to unicast packet and then send it to the DHCP Server.

What is a DHCP Scope and why is it required

A scope includes information like IP address ranges, Subnet Masks, gateway address, DNS Server etc. This required so that clients would require the necessary information from the server.

What would happen if there are multiple DHCP servers on a network

The clients would trigger DHCP Discover packets and the server which responds first would provide the IP address to the client

How does a client know that a lease has expired and how is it renewed

When a client receives an IP address from the DHCP Server, the lease expiry date and time is provided as part of DHCP options. This is maintained and recorded by the client. Just before the lease expiry time, the client would initiate a renew request to the server for a new lease

The questions and answers are from the ebook 250+ Network Engineer Interview Questions and answers

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How many DHCP Servers can reside on one subnet

It’s unlimited. But if there is a DHCP client, which server would lease out the IP address ? It’s a simple logic. The first server which responds to the DHCP clients IP address request would lease out the IP address.

Why does Active FTP not work when a firewall is involved between the client and the server.

In Active FTP , once the FTP client establishes the connectivity with the server, the server initiates the data connection to the client. This is a TCP connection which is initiated from a system (FTP Server) which resides outside the network. All traffic initiated from outside is considered as untrusted, and the firewall would drop it by default.

How does Passive FTP resolve the issue with firewalls

In Passive FTP, the FTP client initiates the data connection with the server and not the FTP server with the client. Since the client is residing within the network, the connection is considered as trusted by the firewall.

In Active FTP, which TCP port is used for initiating the control channel

Port 21

Can FTP be used on a server which is hosted on the internet

FTP is a TCP/IP protocol. It can be used on any servers or computers which support TCP/IP.

Is a generic ethernet frame modified when STP is configured on the network

STP is a protocol. It has its own frame when configured. So it would not affect a generic ethernet frame on the network.

How many designated ports can be available on a root bridge

All ports on a root bridge are designated ports.

How many root bridges can be available on a STP configured network


If the priority value of the two switches are same, which switch would be   elected as the root bridge

The switch with the lowest mac-address value would be elected as the root bridge

Can STP frames be routed

STP works at layer 2. It cannot be routed.

Is STP unicast, multicast or broadcast


How does a UDP based application know that a packet has been lost in transit

UDP based applications uses a time out mechanism. They would wait for a specific time and then triggers a timeout if the response has not been received.

If a 24 port switch has 3 vlans configured, how many broadcast and collision domains would it have

The switch would have 3 broadcast domains for each of the vlans. Every port on the switch would be a separate collision domain. There would be 24 collision domains.

How does a switch port identify vlan tagged frames

Vlans tags are inserted in the generic ethernet frame with the value of the vlan. Based on this info, the identification is performed. 

Does HTTP protocol have a port number

HTTP protocol does not have a port number. HTTP uses TCP at the transport layer and HTTP based servers use TCP port 80.

HTTP uses TCP port 80. Does this imply that HTTP client and servers use port 80 for communication

HTTP servers use port 80. HTTP clients use dynamic port numbers which are allocated by the operating systems on which they run.

HTTP is stateless, but TCP, which HTTP uses at the transport layer, is a stateful protocol. What does the statement imply

TCP maintains the state of data which is sent and received during a HTTP based communication. It has the capacity to know which section of data has been received or not. But HTTP, as a protocol does not have this capacity and needs to rely on TCP for the same.

The questions and answers are from the ebook 250+ Network Engineer Interview Questions and answers

Buy the Ebook and get answers to 250+ interview questions.