36 TCP/IP Interview questions and answers to clear your interview.
1. What are the components which constitute a TCP/IP socket pair.
A socket pair comprises of source port, destination port, destination IP address and source IP address.
2. How is the source port number in a transport layer header derived
The source port number is derived by the operating system and is unique for every connection.
3. What is the source port number in a TCP packet which is targeted to a Web server.
The source port number would be a random number which is derived by the operating system.
4. Name 3 common fields which is found in TCP and UDP headers
The source port, destination port and checksum are common fields which are part of TCP and UDP headers
5. Which tool can be used to find the list of TCP/IP connections on an operating system.
netstat command is used for identifying the list of TCP/UDP connections on a operating system.
6. Which ports numbers are used initially by a FTP client to establish connectivity with a FTP server.
FTP servers work on TCP port 20 and 21. FTP clients initially establish 3 way handshake connectivity with TCP port 21, which is the control connection. After successful connection, another 3 way handshake is established with TCP port 20 , which is the FTP data port.
7. What is configured on the TCP/IP adapter of a PC for deriving the network address in the routing table.
The network address is derived by Anding operation with the IP address and subnet mask which is configured on the TCP/IP adapter of a PC.
8. What happens when the command ping tcpipguru.com is typed on command prompt.
When a ping is initiated to a website name, a DNS resolution follows which would retrieve the IP address on which the website is hosted. The system would now initiate the ping to the servers IP address (Web server on which tcpipguru.com is hosted), which is received after DNS resolution, following which the servers IP address responds.
9. Which protocol do https servers use at the transport layer.
https servers use TCP at the transport layer. TCP port 443 is used.
10. A user on a PC behind a NAT router access tcpipguru.com. To which IP address would the DNS query packet be sent to from the PC for name resolution.
DNS queries from the PC would be sent to the DNS server IP address which is configured on the PC.
11. Does HTTP commence data transfer before or after the TCP 3 way handshake.
Data transfer commences after TCP 3 way handshake.
12. Name a layer 2 protocol which does not use TCP.
ARP protocol does not use TCP.
13. Which transport layer protocol does ping use
Ping does not use any transport layer headers. It uses ICMP at the network layer.
14. Which protocol does ping and tracert use in common
Ping and tracert uses ICMP at the network layer.
15. Which VPN protocol uses TCP port 1723 for communication
PPTP uses TCP port 1723 for communication.
16. Which field in an IP header does a router look to forward the packet.
Routers look into the destination IP address in the IP header to forward the packet.
17. Is the subnet mask included in the IP or TCP header.
The subnet mask is not included in any of the headers.
18. Can a PC ping its gateway if a default gateway is not configured on it.
A PC can ping its default gateway if the address is not configured on the TCP/IP adapter. This is because, the IP address of default gateways are also part of the same network.
19. Which protocol does OSPF use at the transport layer.
OSPF does not use any protocol at the transport layer.
20. Name two protocols which are broadcast in nature.
DHCP and ARP are broadcast in nature. DHCP Discover packets are broadcast packets sent by DHCP clients to locate a DHCP server. Similarly ARP request packets are broadcast packets sent by systems on the network to find the mac-address of a Peer.
21. Give two reasons as to why a ping reply is not received from a remote PC on the LAN.
The remote PC could be shutdown or a firewall would be blocking the packets on it.
22. Can two systems connected to a switch ping each other if they are residing on different networks.
A switch does not have the capacity for allowing two systems on different networks to communicate. So the ping would fail.
23. Which protocol is used by a browser to establish connectivity with a website.
The browser uses http at the application layer, which internally uses TCP for establishing 3 way connectivity with the Web server on which the website is hosted
24. Which field in an ethernet frame does a router look to check if it is intended for itself.
Routers look into the destination mac-address in the ethernet frame to check if the frame is intended for itself.
25. Name a common field which is available in a TCP and IP header.
Checksum is a common field which is available in both the headers.
26. What tool can be used to check if an ARP reply is being received from a peer for a corresponding ARP request.
Protocol analyzers like wireshark, tcpdump can be used for the purpose.
27. Which protocol should be blocked to deny incoming ping packets to a firewall.
Ping uses ICMP protocol. So it should be blocked to deny incoming ping packets.
28. Which field in an IP header is re-written with the IP address of a NAT router.
The source IP address field in the IP header is re-written with the IP address of the NAT router.
29. Which port should be configured on a firewall to allow remote desktop connections to a Windows desktop on the LAN network.
Remote desktop servers work on TCP port 3389. Remote desktop clients connect to this port for establishing connectivity to the server. So the port should be configured on the firewall.
30. Name a routing protocol which is broadcast and multicast
RIP has both broadcast and multicast versions. RIP v1 is broadcast and v2 is multicast.
31. Which protocol does a router use to inform the source that a destination is unreachable.
Routers use ICMP protocol for the source. ICMP protocol with the required type and code value is inserted in the ICMP header.
32. What type of packets are not forwarded by a router
Routers do not forward broadcast packets.
33. A user clicks on a link on a website to download a file. Which application layer protocol is being used.
When a user clicks on a website link to download a file, http is being used
34. Name a protocol which use dual ports for communication
FTP uses TCP port 20 and 21 for communication.
35. Which field in an IP header is decremented when the packet crosses a router
TTL values in IP headers are decremented every time an IP packet crosses a router. When TTL value reaches 1, the packet is not forwarded to the next hop.
36. Which field in a TCP/IP packet would a hub look into to make a forwarding decision.
Hubs do not have the intelligence to understand or read TCP/IP Packets.