What Is Packet Loss?
Packet loss occurs when one or more packets of data travelling across a network fail to reach their destination. It's one of the most disruptive network problems — causing slow page loads, dropped connections, and failed requests even when a site is technically online.
How the internet sends data
Data on the internet is broken into small chunks called packets. A single webpage request might involve hundreds of packets travelling independently across the network, potentially taking different routes, before being reassembled in order at the destination. This design makes the internet resilient — if one route is blocked, packets find another. Learn more about packet loss on Wikipedia.
But packets can still go missing. When a packet is lost, the protocol (TCP) detects the gap and requests a retransmission. This adds delay. If many packets are lost, retransmissions pile up, and connections become slow or completely stall.
What causes packet loss
Network congestion
The most common cause. When a router receives more data than it can process, it drops packets rather than letting them queue indefinitely. This happens most often on busy network links during peak traffic.
Faulty hardware
Damaged cables, failing network interface cards, or buggy routers and switches can corrupt or drop packets. Even a loose ethernet cable can introduce intermittent packet loss.
Wireless interference
Wi-Fi signals compete with other wireless signals, appliances, and physical obstructions. Interference causes packets to be corrupted in transit, which the receiving device treats as if they never arrived.
Overloaded servers
A server that can't keep up with incoming requests may drop packets at the network layer before they're even processed. This is often a symptom of a DDoS attack or a sudden traffic surge.
ISP or transit network issues
Problems within your ISP's network, or between transit networks that carry your traffic, can cause packet loss on specific paths. This is common during peering disputes or infrastructure incidents.
Firewall rules
Overly aggressive firewalls or rate-limiting rules can drop packets that match certain patterns, even from legitimate users. This can look identical to network packet loss from the outside.
How to detect packet loss
The classic tool for detecting packet loss is ping. It sends a series of packets to a host and reports how many were returned. A result like "4 packets transmitted, 3 received, 25% packet loss" tells you that one in four packets didn't come back.
For a more detailed picture, traceroute (or tracert on Windows) shows each hop between your device and the destination, with latency at each step. A jump in packet loss at a particular hop points to where the problem is occurring in the network path.
Packet loss vs. a downed server
Both packet loss and a downed server can make websites appear unavailable, but they're fundamentally different problems. A downed server means the destination is simply not responding. Packet loss means some data is being dropped along the path — the server may be healthy, but communication is unreliable.
With severe packet loss (above ~10%), TCP connections struggle to maintain reliability. Pages load partially, timeout errors increase, and connections drop. At 100% packet loss on a path, the result is indistinguishable from a server being offline from your perspective — even though the site may be perfectly reachable from other networks.
This is why SiteAlive checks sites from its own servers independently. If our servers can reach a site but you can't, packet loss or a routing fault on your network path is a likely cause — not a server outage.