CompTIA Exam Prep - ITF+, A+, Network+, Security+, CySA+

A cyclic redundancy check is a short value computed from a block of data and sent alongside it. The receiver recomputes the value and compares. If they differ, the data changed in transit and the frame is discarded. Every Ethernet frame carries one, and on the Network+ exam the useful knowledge is less about the mathematics than about what a rising CRC error counter is telling you. How it works CRC treats the data as a large binary number and divides it by a fixed generator polynomial, keeping the remainder. That remainder is transmitted with the data. The receiver performs the same division over the data and the remainder together; a result of zero means no detected error. It is chosen over a simple checksum because it catches the error patterns that actually occur on physical links. A CRC-32 detects all single-bit errors, all double-bit errors, any odd number of bit errors, and every burst error shorter than 33 bits — and burst errors are exactly what electrical noise and ma...

Two boundaries, two different devices, and one exam question that appears in some form on nearly every networking certification: a switch separates collision domains, a router separates broadcast domains. Everything else follows from understanding why. Collision domains A collision domain is a segment where two devices transmitting simultaneously interfere with each other. This was the defining constraint of shared-media Ethernet, where a hub repeated every signal to every port and CSMA/CD detected collisions and backed off. Switches ended that. Each switch port is its own collision domain, and with full-duplex operation there are no collisions at all — separate transmit and receive paths mean simultaneous transmission is normal rather than a fault. A twenty-four port switch has twenty-four collision domains, and in practice the concept is historical outside of exam questions and half-duplex troubleshooting. It still matters in one live scenario: a duplex mismatch reintroduc...

Distance vector routing protocols learn routes from their neighbours and pass them on. That works until a network goes down and a router learns about the dead destination from a neighbour it originally told — at which point two routers can point at each other indefinitely, incrementing the hop count and forwarding packets in a circle. Split horizon and its companions exist to stop exactly that. Counting to infinity The failure works like this. Router A is connected to network X. A tells B about X at one hop; B tells C at two hops. Network X fails and A removes the route. Before A can inform B, B advertises its own route to X back toward A. A now believes it can reach X through B at three hops, and tells B so. B updates to four, A to five, and the count climbs while packets for X loop between them. The base defence is a maximum metric — sixteen hops is unreachable in RIP — which caps the damage but does not prevent it. The loop still forms and still wastes time con...