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JNCIA-Junos practice questions

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Question 1Networking Fundamentals

Which OSI layer is responsible for logical addressing and choosing a path between networks?

  1. Data link layer
  2. Session layer
  3. Transport layer
  4. Network layer Correct answer.

Explanation

Layer 3, the network layer, carries logical (IP) addresses and makes forwarding decisions between networks. Layers 1 and 2 deal with the physical medium and local framing, while Layer 4 handles end-to-end delivery.

Question 2Networking Fundamentals

Which transport layer protocol provides reliable, connection-oriented delivery with acknowledgements and retransmissions?

  1. ARP
  2. ICMP
  3. TCP Correct answer.
  4. UDP

Explanation

TCP opens a connection with a handshake, numbers segments, acknowledges data received, and retransmits lost segments, which makes it reliable. UDP offers no such guarantees, and ICMP and ARP are not transport protocols.

Question 3Networking Fundamentals

How many usable host addresses does a subnet with a /26 prefix provide?

  1. 64
  2. 30
  3. 62 Correct answer.
  4. 32

Explanation

A /26 leaves 6 host bits, so the block contains 2^6 = 64 addresses. The all-zero network address and the all-ones broadcast address cannot be assigned to hosts, leaving 62 usable addresses.

Question 4Networking Fundamentals

What is the length of a MAC address?

  1. 32 bits
  2. 128 bits
  3. 64 bits
  4. 48 bits Correct answer.

Explanation

MAC addresses are 48 bits long and are usually written as six hexadecimal octets, such as 00:1b:44:11:3a:b7. The 64-bit value is used for modified EUI-64 interface identifiers in IPv6.

Question 5Networking Fundamentals

What is the default subnet mask for a class B IPv4 address?

  1. 255.0.0.0
  2. 255.255.0.0 Correct answer.
  3. 255.255.240.0
  4. 255.255.255.0

Explanation

Class B addresses use the first two octets for the network portion, giving a default mask of 255.255.0.0, or /16. The classful default is used when no subnetting has been applied.

Question 6Networking Fundamentals

Which address belongs to an RFC 1918 private range?

  1. 192.169.10.5
  2. 11.16.0.5
  3. 172.20.10.5 Correct answer.
  4. 172.32.10.5

Explanation

The private blocks are 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 through 172.31.255.255), and 192.168.0.0/16. The address 172.20.10.5 falls inside the 172.16.0.0/12 range.

Question 7Networking Fundamentals

What is the broadcast address for the network 192.168.10.0/24?

  1. 192.168.10.254
  2. 192.168.10.0
  3. 192.168.10.255 Correct answer.
  4. 192.168.11.255

Explanation

The broadcast address is formed by setting every host bit to 1. With 8 host bits in a /24, the last octet becomes 255, giving 192.168.10.255.

Question 8Networking Fundamentals

Which class of IPv4 address covers the range 224.0.0.0 through 239.255.255.255?

  1. Class C
  2. Class A
  3. Class D Correct answer.
  4. Class B

Explanation

Class D space begins with the bit pattern 1110 and spans 224.0.0.0 to 239.255.255.255. These addresses are reserved for multicast groups and are not assigned to individual hosts.

Question 9Networking Fundamentals

Which protocol learns the MAC address that corresponds to a known IPv4 address on the local link?

  1. DHCP
  2. ARP Correct answer.
  3. DNS
  4. RARP

Explanation

ARP broadcasts a request asking which device owns a given IPv4 address and caches the reply so frames can be built with the correct destination MAC. Without ARP, a host could not place a destination MAC in the frame header.

Question 10Networking Fundamentals

Which sequence describes how a TCP session is established?

  1. SYN-ACK, SYN, ACK
  2. SYN, ACK, SYN-ACK
  3. ACK, SYN, SYN-ACK
  4. SYN, SYN-ACK, ACK Correct answer.

Explanation

The client sends a SYN, the server replies with a SYN-ACK that acknowledges the client's sequence number and supplies its own, and the client completes the handshake with an ACK. Only after this exchange does data transfer begin.

Question 11Networking Fundamentals

What is the role of the default gateway configured on a host?

  1. It hides private addresses from the Internet
  2. It routes traffic to remote subnets Correct answer.
  3. It resolves host names for the host
  4. It assigns addresses to local hosts

Explanation

When the destination lies outside the host's own subnet, the host sends the frame to the default gateway, which forwards the packet toward the destination network. The gateway must be a router interface on the host's local subnet.

Question 12Networking Fundamentals

How many subnets are created by subnetting 192.168.1.0/24 into /27 networks?

  1. 4
  2. 8 Correct answer.
  3. 2
  4. 16

Explanation

Moving from /24 to /27 borrows 3 bits of the host portion. The number of subnets is 2^3, which equals 8, and each subnet holds 32 addresses.

Question 13Networking Fundamentals

What is the decimal value of the binary octet 11000000?

  1. 192 Correct answer.
  2. 240
  3. 224
  4. 128

Explanation

The first two bits from the left represent 128 and 64, and both are set, so the value is 128 + 64 = 192. The remaining six bits are zero and add nothing.

Question 14Networking Fundamentals

What is the network address for the host 192.168.5.130/26?

  1. 192.168.5.130
  2. 192.168.5.192
  3. 192.168.5.0
  4. 192.168.5.128 Correct answer.

Explanation

A /26 uses a block size of 64, so the subnets begin at .0, .64, .128, and .192. The address .130 lies in the block that starts at .128, making .128 the network address.

Question 15Networking Fundamentals

Which transport layer protocol is connectionless and performs no retransmission of lost data?

  1. UDP Correct answer.
  2. FTP
  3. TCP
  4. SCTP

Explanation

UDP sends datagrams without establishing a session and without acknowledgements, so lost datagrams are not retransmitted by the protocol. Applications that use UDP must handle reliability themselves if they need it.

Question 16Networking Fundamentals

Which Ethernet frame field contains a CRC used to detect transmission errors?

  1. Destination MAC address
  2. EtherType field
  3. Source MAC address
  4. Frame check sequence Correct answer.

Explanation

The frame check sequence holds a cyclic redundancy check computed over the frame contents. A receiving NIC recalculates the CRC and discards the frame if the result does not match.

Question 17Networking Fundamentals

A 24-port switch has one host connected to each port, and all ports belong to a single VLAN. How many collision domains exist?

  1. 12
  2. 1
  3. 24 Correct answer.
  4. 48

Explanation

Each switch port is an independent collision domain because the switch buffers and forwards frames rather than repeating electrical signals. The whole VLAN remains a single broadcast domain regardless of port count.

Question 18Networking Fundamentals

Which ICMP message pair does the ping utility rely on?

  1. Redirect messages
  2. Time exceeded messages
  3. Echo request and reply Correct answer.
  4. Destination unreachable messages

Explanation

Ping sends an ICMP echo request and waits for the corresponding echo reply to confirm reachability and measure round-trip time. The other message types report specific error or routing conditions rather than testing reachability directly.

Question 19Networking Fundamentals

What is the primary function of DHCP?

  1. Maps IP addresses to MAC addresses
  2. Assigns IP settings to hosts Correct answer.
  3. Maps names to IP addresses
  4. Encrypts management sessions

Explanation

DHCP dynamically supplies an IP address, subnet mask, default gateway, and DNS servers to clients, which reduces manual configuration errors. The server leases addresses for a defined period and tracks which addresses are in use.

Question 20Networking Fundamentals

Which address is reserved for testing the local TCP/IP stack on a device?

  1. 192.168.0.1
  2. 10.0.0.1
  3. 127.0.0.1 Correct answer.
  4. 169.254.1.1

Explanation

The 127.0.0.0/8 block is reserved for loopback, and 127.0.0.1 is the conventional loopback address. Traffic sent to it never leaves the device, so it verifies that the local stack is functioning.

Question 21Networking Fundamentals

Given the network 172.16.32.0/20, what is the last usable host address in the subnet?

  1. 172.16.47.255
  2. 172.16.32.254
  3. 172.16.47.254 Correct answer.
  4. 172.16.48.254

Explanation

A /20 uses a block size of 16 in the third octet, so the subnet runs from 172.16.32.0 to 172.16.47.255. The broadcast address is 172.16.47.255, which makes 172.16.47.254 the final assignable host address.

Question 22Networking Fundamentals

Which single prefix summarizes the four networks 10.1.0.0/24, 10.1.1.0/24, 10.1.2.0/24, and 10.1.3.0/24?

  1. 10.1.0.0/23
  2. 10.1.0.0/24
  3. 10.1.0.0/21
  4. 10.1.0.0/22 Correct answer.

Explanation

Four contiguous /24 blocks require two host bits to be aggregated, which reduces the prefix length from 24 to 22. The resulting 10.1.0.0/22 spans exactly 10.1.0.0 through 10.1.3.255.

Question 23Networking Fundamentals

Host A is 10.0.0.30/27 and Host B is 10.0.0.40/27. Do the two hosts share a subnet?

  1. Yes, same /27 subnet
  2. Yes, same /24 network
  3. No, these are broadcast addresses
  4. No, different subnets Correct answer.

Explanation

A /27 uses a block size of 32, so subnets begin at .0, .32, .64, and so on. Host A sits in 10.0.0.0/27 while Host B sits in 10.0.0.32/27, so they are on different subnets and need a router to communicate.

Question 24Networking Fundamentals

In what order does a sending host encapsulate data as it moves down the protocol stack?

  1. Segment, data, packet, frame
  2. Data, packet, segment, frame
  3. Data, segment, packet, frame Correct answer.
  4. Frame, packet, segment, data

Explanation

Application data is first wrapped by a transport header to form a segment, then by an IP header to form a packet, then by an Ethernet header and trailer to form a frame. The frame is finally transmitted as bits on the medium.

Question 25Networking Fundamentals

A host with address 192.168.1.100/26 sends a packet to 192.168.1.200. What does the host do at Layer 2?

  1. It uses no ARP at all
  2. It floods to the broadcast MAC
  3. It ARPs for the gateway MAC Correct answer.
  4. It ARPs for the destination MAC

Explanation

With a /26 mask, 192.168.1.100 sits in the 192.168.1.64/26 block while 192.168.1.200 sits in 192.168.1.192/26. Because the destination is off-subnet, the host builds the frame with the default gateway's MAC address and ARPs for that address instead of the destination.

Question 26Networking Fundamentals

At which OSI layer does a router primarily make forwarding decisions based on logical addressing?

  1. Transport layer (Layer 4)
  2. Data link layer (Layer 2)
  3. Network layer (Layer 3) Correct answer.
  4. Physical layer (Layer 1)

Explanation

Routers build a path between different networks by examining a packet's destination logical (IP) address and selecting the best matching route, which is a Layer 3 function. Layer 2 addresses are rewritten hop by hop, but the forwarding decision in a router is driven by the Layer 3 destination.

Question 27Networking Fundamentals

What is the main purpose of the Time-to-Live (TTL) field in an IPv4 header?

  1. It marks the priority a router should give to the packet
  2. It records how long a packet has waited in a router's output queue
  3. It carries the timestamp used to calculate round-trip time
  4. It limits how many hops a packet can traverse Correct answer.

Explanation

Each router decrements the TTL by one and discards the packet when the value reaches zero, which prevents packets from circulating endlessly when a routing loop exists. The discarding router normally returns an ICMP Time Exceeded message to the source.

Question 28Networking Fundamentals

A design requires each subnet to support at least 50 usable host addresses while wasting as little address space as possible. Which prefix length meets the requirement?

  1. /25
  2. /28
  3. /27
  4. /26 Correct answer.

Explanation

Host bits are calculated as 2^n − 2, where n is the number of host bits. A /26 leaves 6 host bits, giving 62 usable addresses, which satisfies 50 with the least waste; a /27 provides only 30 usable addresses and a /25 provides 126, which is more space than needed.

Question 29Networking Fundamentals

A switch receives a unicast frame whose destination MAC address is not present in its MAC address table. How does the switch handle the frame?

  1. It holds the frame until the destination address is learned
  2. It floods the frame out of all other ports in the same VLAN Correct answer.
  3. It sends the frame back out the port on which it arrived
  4. It drops the frame and notifies the sender with an ICMP error

Explanation

When the destination MAC address is unknown, the switch treats the frame as unknown unicast and floods it to every other port in the same VLAN so the destination can still be reached. The destination's reply then teaches the switch the correct MAC-to-port mapping for future frames.

Question 30Networking Fundamentals

Host A must send an IP packet to Host B, which is on the same subnet. Host A has no entry for Host B in its ARP cache. What does Host A do first?

  1. It performs a reverse DNS lookup to translate the IP address into a hardware address.
  2. It sends an ICMP echo request to the destination to discover its hardware address.
  3. It broadcasts an ARP request asking which device owns the destination IP address. Correct answer.
  4. It forwards the packet to the default gateway so the gateway can resolve the address.

Explanation

When the destination is on the same subnet but the frame's destination MAC address is unknown, the sender uses ARP: it broadcasts a request containing the target IP address, and the owner of that address replies with its MAC address. The resolved mapping is then cached so subsequent frames can be built directly.

Question 31Junos OS Fundamentals

Which prompt indicates that the user is currently in Junos configuration mode?

  1. user@router$
  2. user@router>
  3. user@router# Correct answer.
  4. root@router%

Explanation

The Junos CLI uses the greater-than sign for operational mode and the hash sign for configuration mode. The prompt changes as soon as the user enters or leaves configuration mode.

Question 32Junos OS Fundamentals

Which command is used to move from operational mode into configuration mode?

  1. edit
  2. configure Correct answer.
  3. set
  4. commit

Explanation

Configuration mode is entered by typing configure at the operational mode prompt. The prompt then changes and the candidate configuration becomes editable.

Question 33Junos OS Fundamentals

What is the effect of the commit command in Junos?

  1. It activates the candidate configuration as the active configuration Correct answer.
  2. It validates the candidate configuration without activating it
  3. It loads the previously committed configuration into the candidate
  4. It exports the candidate configuration to a text file on the device

Explanation

Junos edits a candidate configuration, and the commit command promotes that candidate to become the active configuration. Each commit is also archived, which is what makes rollback possible.

Question 34Junos OS Fundamentals

An administrator has made changes in configuration mode but has not committed them. Which command discards all of those uncommitted changes?

  1. commit confirmed 1
  2. commit check
  3. rollback 1
  4. rollback 0 Correct answer.

Explanation

Rollback 0 replaces the candidate configuration with the currently active configuration, so every uncommitted edit is lost. Rollback 1 loads the previous committed configuration, which is a different action.

Question 35Junos OS Fundamentals

From operational mode, which command displays the currently active configuration?

  1. show configuration Correct answer.
  2. show config active
  3. show candidate-config
  4. show system commit

Explanation

In operational mode, show configuration displays the active configuration; the same command in configuration mode displays the candidate instead. This distinction matters when troubleshooting whether a change was really committed.

Question 36Junos OS Fundamentals

In the Junos architecture, which component handles the forwarding of transit traffic?

  1. The Routing Engine
  2. The Packet Forwarding Engine Correct answer.
  3. The management Ethernet interface
  4. The console port

Explanation

Junos separates the control plane from the forwarding plane. The Routing Engine runs routing protocols and builds the forwarding table, while the Packet Forwarding Engine uses that table to forward packets.

Question 37Junos OS Fundamentals

Which command verifies the syntax of the candidate configuration without making it active?

  1. commit check Correct answer.
  2. commit comment
  3. commit and-quit
  4. commit synchronize

Explanation

Commit check parses the candidate configuration and reports errors, but the active configuration remains unchanged. It is commonly used to validate a large change before activating it.

Question 38Junos OS Fundamentals

An administrator enters commit confirmed 5 and then loses connectivity. What happens if the commit is never confirmed?

  1. The changes remain active and the session closes normally
  2. The configuration reverts to the last committed version Correct answer.
  3. The device reloads the factory default configuration
  4. The candidate configuration is discarded automatically

Explanation

Commit confirmed activates the configuration but starts a timer; unless a confirming commit is issued before the timer expires, Junos automatically returns to the previously committed configuration. This protects against changes that cut off management access.

Question 39Junos OS Fundamentals

Which command displays the differences between the candidate configuration and the active configuration?

  1. show | match
  2. show | count
  3. show | compare Correct answer.
  4. show | display set

Explanation

In configuration mode, show | compare lists the pending changes with plus and minus markers against the active configuration. It is the standard way to review edits before committing them.

Question 40Junos OS Fundamentals

Which statement associates a configuration group with a section of the configuration hierarchy?

  1. group-apply
  2. include-groups
  3. inherit-groups
  4. apply-groups Correct answer.

Explanation

Placing apply-groups under a hierarchy level causes the statements defined in that group to be inherited at that level. Explicitly configured statements at the same level override inherited ones.

Question 41Junos OS Fundamentals

Which pipe filter displays configuration output as a series of set commands?

  1. | compare
  2. | display set Correct answer.
  3. | display inheritance
  4. | match set

Explanation

The display set filter renders the configuration in the same flat format produced by the set command, which is convenient for copying statements into scripts. It does not change what is configured, only how the output is presented.

Question 42Junos OS Fundamentals

What does the top command do while in configuration mode?

  1. Commits the candidate configuration and exits configuration mode
  2. Moves directly to the top of the configuration hierarchy Correct answer.
  3. Deletes the most recent change made to the candidate configuration
  4. Moves up exactly one level in the configuration hierarchy

Explanation

The top command jumps straight to the [edit] level regardless of how deep the user is in the hierarchy. The up command, in contrast, moves only one level toward the top.

Question 43Junos OS Fundamentals

Which pipe filter removes lines that contain a specified string from command output?

  1. | except Correct answer.
  2. | find
  3. | hold
  4. | match

Explanation

The except filter suppresses any line matching the given expression, leaving the rest of the output intact. It is the inverse of the match filter, which keeps only matching lines.

Question 44Junos OS Fundamentals

Which command runs an operational mode command from within configuration mode?

  1. operational
  2. run Correct answer.
  3. do
  4. exec

Explanation

Prefixing a command with run executes it as an operational mode command without leaving configuration mode. This lets an administrator check interface state or routes while editing the configuration.

Question 45Junos OS Fundamentals

Which command activates the candidate configuration and returns the user to operational mode in one step?

  1. commit check
  2. commit and-quit Correct answer.
  3. commit comment
  4. commit synchronize

Explanation

Commit and-quit performs a normal commit and then exits configuration mode, which is convenient when the change is complete. Commit alone activates the changes but leaves the user in configuration mode.

Question 46Junos OS Fundamentals

While at the top of the configuration hierarchy, which command adds an IPv4 address to interface ge-0/0/0 unit 0?

  1. set interfaces ge-0/0/0 address 10.0.0.1/24
  2. edit interfaces ge-0/0/0 unit 0 family inet address 10.0.0.1/24
  3. set interfaces ge-0/0/0 unit 0 family inet address 10.0.0.1/24 Correct answer.
  4. add interfaces ge-0/0/0 unit 0 family inet address 10.0.0.1/24

Explanation

The set command accepts the full path from the current hierarchy level, so the complete unit and family inet path must be supplied. It creates the statement if it does not already exist.

Question 47Junos OS Fundamentals

Which command displays the commit history, including the user and time of each commit?

  1. show commit log
  2. show system commit Correct answer.
  3. show system rollback
  4. show configuration history

Explanation

Show system commit lists the most recent commits with sequence numbers, timestamps, and the user who performed each one. Those sequence numbers are the values used with the rollback command.

Question 48Junos OS Fundamentals

Which command saves the current configuration as the rescue configuration?

  1. set system rescue configuration
  2. save configuration rescue
  3. request system configuration rescue save Correct answer.
  4. commit configuration rescue

Explanation

The rescue configuration is a known-good snapshot stored separately from the rollback history and is created with request system configuration rescue save. It can later be restored even if the normal rollback history has been exhausted.

Question 49Junos OS Fundamentals

On a device with two Routing Engines, which commit option activates the configuration and copies it to the backup Routing Engine?

  1. commit full
  2. commit synchronize Correct answer.
  3. commit and-quit
  4. commit confirmed

Explanation

Commit synchronize commits the candidate on the local Routing Engine and replicates the active configuration to the other Routing Engine. Without it, the two Routing Engines can hold different configurations.

Question 50Junos OS Fundamentals

How does the load override command differ from the load replace command?

  1. It replaces only the hierarchy levels that appear in the file
  2. It replaces the entire candidate configuration with the file contents Correct answer.
  3. It appends the file contents to the end of the candidate configuration
  4. It merges the file contents into the existing candidate configuration

Explanation

Load override discards the whole candidate configuration and substitutes the contents of the file, so any statement not present in the file disappears. Load replace, by contrast, only substitutes the hierarchies that the file actually contains.