Why does the first ping in a series often time out even when the host is reachable?

Question

Accepted Answer

Because address resolution (ARP for IPv4 or Neighbor Discovery for IPv6) usually has to happen before the first ICMP Echo Request can be sent, and that lookup can take longer than the ping timeout.
First-ping timeout:
It is common for the first ping to time out if address resolution needs to be performed before the ICMP Echo Request can actually be transmitted:

IPv4: the sender must first run ARP to learn the destination (or gateway) MAC address.
IPv6: the sender must first run Neighbor Discovery (NS/NA) to learn the MAC.

While that resolution completes, the first echo request may not go out in time, so it reports a timeout. Subsequent pings reuse the cached mapping and succeed.
Key insight: A single failed first ping followed by successful replies is normal and does not indicate a connectivity problem — judge reachability by the overall success rate, not the first packet.
Go deeper:

Neighbor Discovery Protocol — Wikipedia — the NS/NA resolution (and IPv4 ARP equivalent) that must finish before the first echo can be sent.