CCNA Practice Test - 50

On Cisco routers, Network Address Translation (NAT) uses Access Control Lists (ACLs) to define which IP addresses are eligible for translation. The ACL acts as a match criterion that specifies the 'inside local' addresses that should be translated to 'inside global' addresses. Routing protocols, interfaces, and ARP cache are not used for this purpose.

For a /19 network (172.16.128.0 to 172.16.159.255), we need to address 500+ hosts efficiently. A /22 provides 1022 usable addresses (2^10-2), which is sufficient with minimal waste. Smaller masks (/19-/21) waste addresses; /23 only gives 510 hosts, which is insufficient. Branch4 likely needs 500-1000 hosts.

Standard access control lists should be placed close to the destination, not the source. This is because standard ACLs only filter based on source IP and cannot distinguish between permitted and denied traffic streams. Placing them near the source might block legitimate traffic destined for other networks.

TCP provides reliable, ordered delivery through sequence numbers in every segment. When packets arrive out of order, the receiving TCP buffer stores them until missing segments arrive, then reassembles everything into the original message. This reassembly happens automatically at the destination before data is delivered to the application.

The transport input command on VTY lines controls which protocols are allowed for remote access. If it is set to only SSH (as implied by the SSH configuration), Telnet will be blocked. To permit both SSH and Telnet, the command must explicitly allow both protocols with transport input ssh telnet.

The wildcard mask 0.0.3.255 corresponds to a /22 subnet mask (255.255.252.0), which covers the address range 192.168.4.0 through 192.168.7.255. Combined with eq telnet (TCP port 23), this ACL permits only Telnet traffic originating from anywhere in this /22 network to any destination.

The line console 0 command enters console line configuration mode. Setting a password and enabling login secures the console port so that anyone attempting console access must provide this password. This is a basic but essential security measure for restricting physical access to the switch.

When multiple routing protocols learn paths to the same destination, the router chooses the path with the lowest administrative distance. EIGRP has an administrative distance of 90, while RIP has 120. Therefore, even with equal costs, the EIGRP path is preferred and installed in the routing table.

Link-state routing protocols like OSPF and IS-IS use Dijkstra shortest path first (SPF) algorithm to calculate the most efficient path to all reachable networks. The protocol builds a complete topology map (link-state database) and runs Dijkstra algorithm to compute the shortest path tree from itself to all destinations.

By default, Cisco switches allow all VLANs (1-4094) across a trunk port. The default trunk configuration does not restrict VLAN traffic unless explicitly configured with the switchport trunk allowed vlan command. This allows all VLANs, including the extended range VLANs, to traverse the trunk link.

RIP uses a 180-second invalid timer. If a route is not refreshed (no update received) within 180 seconds, it is marked invalid and the metric is set to 16 (unreachable). This is followed by a 240-second flush timer before the route is completely removed from the routing table.

No cdp enable disables CDP for a particular interface

Encapsulation at Layer 2 wraps network layer packets into frames with source and destination MAC addresses, error checking, and frame delimiters. This formatting is necessary for data to be properly transmitted across the physical medium. The data link layer controls how data enters and exits the network media.