Error map
arp -a shows (incomplete) Networking: `arp -n` shows (incomplete) / Destination Host Unreachable — no ARP reply from the next hop
Networking · layer 2, Network
Centre: this error. First ring: errors it is linked with (12). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (12)
Networking: Network cable unplugged / `ip link` shows NO-CARRIER state DOWN / Ethernet: Network cable unplugged
The network card sees no electrical link, so nothing above it (IP, DNS, browser) can work.
Networking: Windows has detected an IP address conflict / Duplicate address detected by 00:aa:bb:cc:dd:ee
Two devices on the same network are using the same IP address, so traffic reaches the wrong one or neither.
Networking: wlan0: WPA: 4-way handshake failed - pre-shared key may be incorrect / WRONG_KEY
The Wi-Fi association started but key negotiation failed, almost always a wrong password.
Networking: Duplex mismatch — late collisions, runts and CRC errors on the switch port
One side runs full duplex and the other half duplex, so the link works but is slow and drops packets.
Networking: %CDP-4-NATIVE_VLAN_MISMATCH / device on the wrong VLAN can't reach the gateway
A port or trunk carries a different VLAN than expected, so hosts silently land on the wrong network.
Networking: %PM-4-ERR_DISABLE: psecure-violation error detected on Gi0/5, putting Gi0/5 in err-disable state
The switch shut the port down because a security or protection feature detected a violation.
Networking: Two hosts on one LAN can't talk — subnet mask or network address mismatch
The hosts disagree about which addresses are local, so one tries to route traffic that should go straight across the LAN.
Networking: Port forwarding configured but the port stays closed — carrier-grade NAT (CGNAT) or double NAT
Your router forwards the port correctly, but another NAT in front of it (your ISP or a second router) blocks inbound connections.
Networking: netstat -su: packet receive errors, receive buffer errors — UDP datagrams dropped by a full socket buffer
UDP packets arrive faster than the application reads them, so the kernel throws the extras away.
Networking: kernel: nf_conntrack: table full, dropping packet — new connections fail on a Linux firewall/NAT host
The connection-tracking table is full, so the kernel drops packets for any new connection.
Networking: kernel: nfs: server nas01 not responding, still trying / nfs: server nas01 OK
An NFS client can't get replies from the server, so processes using the mount hang in uninterruptible sleep.
Networking: dhcpd: DHCPDISCOVER from aa:bb:cc:dd:ee:ff via eth0: network 192.168.1.0/24: no free leases
The DHCP server has no unused addresses left in the scope.
Two steps away (27)
connect: Network is unreachable (ENETUNREACH)
The host has no route to the destination network — no default gateway, interface down, or trying IPv6 without IPv6 connectivity.
Chrome: No internet — ERR_INTERNET_DISCONNECTED (You are not connected to the internet)
Chrome sees no working network connection on the device, so it cannot load any page.
Networking: No Wi-Fi networks found — the Wi-Fi adapter is missing, disabled or its driver failed
The operating system doesn't see a working wireless adapter, so no networks can be listed.
Networking: Windows: Default gateway is not available / Diagnose Connection Problems: DHCP not enabled
The PC can't reach its router, so it has no path out of the local network.
Networking: IPv4 address 169.254.x.x (Autoconfiguration / APIPA) — the PC couldn't get an address from DHCP
With no DHCP answer, the computer gave itself a link-local 169.254.x.x address, which only works on the local segment.
Networking: dhclient: DHCPDISCOVER on eth0 ... No DHCPOFFERS received. No working leases in persistent database - sleeping.
The client broadcast a DHCP request and no server answered.
Browser: This network requires you to sign in — captive portal (hotel, airport, café Wi-Fi) blocking HTTPS sites
The Wi-Fi needs a login or acceptance page first; until you complete it, secure sites fail with certificate or connection errors.
Networking: Wi-Fi keeps disconnecting / signal -80 dBm / wlan0: deauthenticating by local choice (reason=3)
The wireless link is too weak or too noisy to hold, so the client drops and rejoins repeatedly.
Networking: connect: Cannot assign requested address — ephemeral port range exhausted by TIME_WAIT connections
The machine ran out of local ports for new outgoing connections to the same destination.
Networking: Interface down — SFP transceiver Loss of Signal / no receive light
A fibre or SFP link is down because the transceiver receives no light.
Networking: %SW_MATM-4-MACFLAP_NOTIF: Host is flapping between ports / broadcast storm
A layer-2 loop is making frames circle the network, saturating links and making the whole VLAN crawl.
Networking: netstat -s: segments retransmitted / Wireshark: TCP Retransmission, TCP Dup ACK — packet loss on the path
Packets are being lost, so TCP resends them and throughput collapses.
Networking: %OSPF-5-ADJCHG: Nbr 10.0.0.2 from EXSTART to DOWN — OSPF neighbor stuck in EXSTART/EXCHANGE (MTU mismatch)
Two OSPF routers see each other but never finish exchanging databases, cycling between EXSTART and DOWN.
Networking: ping — Destination Host Unreachable / Destination net unreachable (ICMP type 3)
A router (or your own PC) tells you it has no way to deliver the packet to that address.
Networking: VPN is connected but I can't reach the internal network — missing routes, split tunnel or overlapping subnets
The tunnel is up, but traffic for the office/VPC subnets isn't sent through it or comes back the wrong way.
ssh: connect to host x port 22: Connection timed out
SSH packets get no reply — a firewall/security group drops them, the server is off, or the IP is wrong.
Networking: Server reachable from the internet but not from inside the LAN via its public IP — NAT loopback / hairpin NAT not working
Devices on your own network can't use the router's public address to reach a server that is also on that network.
Networking: nc -vz hangs then times out — firewall silently DROPs packets (REJECT would answer "Connection refused")
A connection attempt gets no answer at all, which points to a firewall dropping the packets rather than a closed port.
Networking: Wireshark: TCP ZeroWindow, TCP Window Full — the receiver can't keep up and tells the sender to stop
The receiving application isn't reading data fast enough, so its TCP buffer filled and the sender must pause.
Networking: ICMP Destination unreachable (Port unreachable) — UDP packet sent to a port where nothing is listening
A host answered a UDP packet with an ICMP message saying no program is listening on that port.
Networking: Handshake for peer did not complete after 5 seconds, retrying (try 2) — WireGuard tunnel never establishes
The WireGuard peers can't complete the cryptographic handshake, so no traffic flows.
DNS not working inside a Kubernetes pod (could not resolve host / nslookup: can't resolve)
Pods can’t resolve service names or external hosts — CoreDNS is down, blocked by NetworkPolicy, or the upstream resolver fails.
Networking: NAT gateway ErrorPortAllocation / SNAT port exhaustion — intermittent outbound connection timeouts
Many instances share few public IPs, and the NAT gateway runs out of source ports for outbound connections.
Networking: TCP: request_sock_TCP: Possible SYN flooding on port 80. Sending cookies. Check SNMP counters.
The server's pending-connection queue filled, so the kernel switched to SYN cookies to survive.
connect: No route to host (EHOSTUNREACH)
The destination host isn’t reachable — it’s down, on another subnet without routing, or a firewall rejects with “host unreachable…
Networking: Windows error 64: The specified network name is no longer available — SMB session dropped
A connection to a Windows file share was cut while in use.
Networking: Clients receive the wrong default gateway or DNS server — a rogue DHCP server (often a home router) answers first
A second DHCP server on the network is giving clients bad settings, so some machines have no internet or odd DNS.
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