LFCA 41 🐧 Gateways and Routing Basics
A device on a network can reach the other devices on the same subnet directly, but it cannot reach the devices on the other subnets without help. That help is the gateway — the router that connects the local network to the outside. Every packet whose destination is not on the local subnet is sent to the gateway, and the gateway forwards it toward the destination. The decision of whether a destination is local or remote is the routing, and the table that holds the decision is the routing table. The gateway is the entry in the table with the default destination, and it is the last resort when no other route matches. This chapter covers the gateway’s role, the routing table’s structure, the default route, the routing decision process, the static and dynamic routes, and the diagnostic tools — the ip route, the ip neigh, the traceroute, and the ping. It builds on the subnet material from LFCA 40 and prepares the ground for the network diagnostics.
Key point: The gateway is the router that connects the local subnet to the other networks. The default route is the routing table entry with the destination 0.0.0.0/0 (or default), and it is the route used when no more specific route matches. The routing table is the list of the routes, and the kernel consults it for every outgoing packet. The routing decision is the longest-prefix match: the kernel compares the packet’s destination against every route, and the route with the longest prefix (the most specific) wins. If no route matches, the packet is dropped, and the error is “Network is unreachable.” The ip route command shows the table, the ip neigh shows the ARP cache, and the ping and the traceroute are the diagnostics.
Why the gateway is needed
A device on a subnet can reach the other devices on the same subnet because they are directly connected. The device sends the packet to the destination’s MAC address, which it finds through the ARP, and the packet is delivered. The direct delivery is the local communication.
The problem with the remote destination. A device on 192.168.1.0/24 cannot reach a device on 192.168.2.0/24 directly, because the two subnets are separate broadcast domains. The device does not have the remote device’s MAC address, and the ARP does not resolve across the subnets. The packet needs the router to forward it.
The gateway’s role. The gateway is the router on the local subnet. The device sends the packet to the gateway’s MAC address, and the gateway forwards it toward the destination. The gateway is the device’s link to the outside, and it is the only address the device needs beyond its own subnet.
Why the device needs the gateway’s address. The device needs the gateway’s IP address to send the packet. The gateway’s address is configured on the device, and it is the gateway’s address on the local subnet. The device’s ARP resolves the gateway’s IP to the gateway’s MAC, and the packet is sent to the MAC.
Why the gateway must be on the local subnet. The gateway’s address must be in the device’s subnet, because the device must reach the gateway through the ARP. A gateway that is not on the local subnet is unreachable, and the configuration is the error.
Why the gateway is the default route. The gateway is the route for the destinations that are not on the local subnet. The route is the default route, and it is the last resort when no more specific route matches.
Why the multiple gateways exist. A device can have the multiple routes, each with the different gateway. The local subnet has the direct route, the default has the gateway, and the specific routes have the specific gateways. The routing table is the list, and the longest prefix is the decision.
Why the gateway is the router. The gateway is a router, and the router has the multiple interfaces. The router’s interface on the local subnet has the gateway’s address, and the router’s other interfaces connect to the other networks. The router forwards the packet from one interface to the other.
Why the gateway is the single point of failure. The device’s connection to the outside depends on the gateway. If the gateway is down, the device cannot reach the remote destinations, and the local communication still works. The gateway’s failure is the isolated, and the local traffic is the unaffected.
The routing table
The routing table is the kernel’s list of the routes. Each route has the destination, the gateway, the interface, and the metric. The kernel consults the table for every outgoing packet.
The structure. Each route has the destination prefix, the gateway (the next hop), the interface, and the metric. The destination is the network prefix, the gateway is the next hop’s address, and the interface is the outgoing interface.
ip route
# default via 192.168.1.1 dev eth0
# 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100The default via 192.168.1.1 dev eth0 is the default route, and the 192.168.1.0/24 dev eth0 is the local subnet route.
Why the local subnet route has no gateway. The local subnet route is the direct, and the packet is sent to the destination’s MAC. The dev eth0 is the interface, and the scope link means the destination is on the link.
Why the default route has the gateway. The default route is the last resort, and the packet is sent to the gateway. The via 192.168.1.1 is the gateway’s address, and the dev eth0 is the interface.
Why the proto kernel matters. The proto kernel means the kernel added the route automatically when the interface’s address was configured. The proto static means the route was added manually, and the proto dhcp means the route came from the DHCP.
Why the src matters. The src 192.168.1.100 is the source address the kernel uses for the packets on the route. The kernel picks the source address from the interface’s address, and the src is the choice.
Why the metric matters. The metric is the route’s preference when the multiple routes have the same prefix. The lower metric wins, and the metric is the tiebreaker.
Why the table can have the multiple entries. The table can have the multiple routes for the different destinations, and the kernel consults the table for every packet. The multiple routes are the different networks, and the longest prefix is the decision.
Why the table is per-namespace. The network namespace has its own routing table, and the containers use the namespaces. The ip netns exec runs the command in the namespace, and the table is the namespace’s.
The default route
The default route is the route with the destination 0.0.0.0/0 (or default). It matches every destination, and it is the last resort when no more specific route matches.
ip route
# default via 192.168.1.1 dev eth0The default is the destination, the via 192.168.1.1 is the gateway, and the dev eth0 is the interface.
Why the 0.0.0.0/0 matches everything. The prefix length is 0, and the prefix is empty. Every address matches the empty prefix, and the route is the catch-all. The default is the shorthand for the 0.0.0.0/0.
Why the default route is the last resort. The longest-prefix match means the specific routes win over the default. The default is consulted only when no more specific route matches. The default is the last resort.
Why the default route is the gateway. The default route’s gateway is the router that connects to the outside. The device sends the packet to the gateway, and the gateway forwards it toward the destination.
Why the default route can be missing. A device with no default route cannot reach the destinations outside its subnets. The error is “Network is unreachable,” and the fix is to add the default route.
sudo ip route add default via 192.168.1.1 dev eth0The ip route add default adds the default route, and the via is the gateway.
Why the default route can be multiple. A device can have the multiple default routes with the different metrics, and the kernel picks the one with the lowest metric. The pattern is the redundant, and the use is the failover.
Why the default route is the common. The default route is the common configuration for the client devices. The device has the local subnet route and the default route, and the two cover the local and the remote communication.
Why the default route should be verified. A wrong default route is the common cause of the network problems. The ip route shows the current, and the ping to the gateway and to a public address verifies the connectivity.
The routing decision
The routing decision is the process the kernel uses to pick the route for the outgoing packet. The process is the longest-prefix match, and the result is the route or the error.
The step 1: the destination. The kernel reads the packet’s destination address, and it compares the address against the routing table.
The step 2: the prefix match. The kernel finds the routes whose prefix matches the destination. The route’s prefix is the network part, and the destination’s network part must match.
The step 3: the longest prefix. The kernel picks the route with the longest prefix (the most specific). The /24 wins over the /16, and the /16 wins over the /0. The longest is the most specific, and the most specific is the correct.
The step 4: the metric. If the multiple routes have the same prefix, the kernel picks the one with the lowest metric. The metric is the tiebreaker, and the lower is the preferred.
The step 5: the gateway or the direct. The route has the gateway or the direct. The gateway’s route sends the packet to the gateway, and the direct route sends the packet to the destination’s MAC. The two are the different paths.
The step 6: the interface. The route has the interface, and the kernel sends the packet out the interface. The interface is the outgoing, and the packet’s path is the interface’s.
The step 7: the source address. The kernel picks the source address from the route’s src or the interface’s address. The source is the packet’s return address, and the destination uses it for the response.
Why the decision is the per-packet. The kernel makes the decision for every outgoing packet, and the decision is the same for the packets with the same destination. The decision is the fast, and the table is the cache.
Why the decision can be the multiple. The kernel can have the multiple routes for the different destinations, and the decision picks the route for the specific destination. The table is the list, and the decision is the selection.
Why the decision is the longest-prefix. The longest-prefix is the most specific, and the most specific is the correct. The general route is the fallback, and the specific route is the preference. The rule is the standard, and the implementation is the kernel’s.
The static and dynamic routes
A route can be the static — the manually added — or the dynamic — the learned from the routing protocol. The two are the different, and the choice depends on the network’s size.
The static route. The static route is the manually added with the ip route add. The route is the fixed, and the administrator is the source.
sudo ip route add 10.0.0.0/8 via 192.168.1.2 dev eth0The 10.0.0.0/8 is the destination, the via 192.168.1.2 is the gateway, and the dev eth0 is the interface. The route is the static, and the addition is the manual.
Why the static route is the explicit. The static route is the administrator’s, and the route is the fixed. The pattern is the small network, and the use is the specific.
The dynamic route. The dynamic route is the learned from the routing protocol — the OSPF, the BGP, the RIP. The route is the automatic, and the protocol is the source.
# The OSPF daemon learns the routes.
ip route
# 10.0.0.0/8 via 192.168.1.2 dev eth0 proto ospf metric 20The proto ospf is the protocol, and the route is the learned. The pattern is the large network, and the use is the dynamic.
Why the dynamic route is the automatic. The dynamic route is the learned from the protocol, and the route is the automatic. The pattern is the large network, and the use is the automatic.
Why the static route is the preference for the small. The static route is the simple, and the small network does not need the protocol. The static is the direct, and the administrator is the control.
Why the two can coexist. The kernel can have the static and the dynamic routes in the same table. The longest prefix is the decision, and the metric is the tiebreaker. The two are the different, and the table is the merged.
Why the route’s persistence matters. The static route added with the ip route add is the temporary, and it is lost at the reboot. The persistent route is in the /etc/network/interfaces or the systemd-networkd‘s configuration. The temporary is the test, and the persistent is the production.
Why the route’s scope matters. The route’s scope is the link, the global, or the host. The link scope is the direct, the global is the gateway, and the host is the single. The scope is the detail, and the route is the general.
The diagnostic tools
The routing diagnostics use the ip route, the ip neigh, the ping, and the traceroute. Each has the specific purpose, and the combination is the diagnosis.
The ip route. The ip route shows the routing table.
ip route
# default via 192.168.1.1 dev eth0
# 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100The default route is the gateway, and the local route is the direct. The two are the essential.
The ip route get. The ip route get shows the route for a specific destination.
ip route get 8.8.8.8
# 8.8.8.8 via 192.168.1.1 dev eth0 src 192.168.1.100 uid 1000The command shows the route the kernel would use for the destination. The via is the gateway, and the src is the source address. The command is the way to verify the routing decision.
The ip neigh. The ip neigh shows the ARP cache, which maps the IP addresses to the MAC addresses on the local network.
ip neigh
# 192.168.1.1 dev eth0 lladdr aa:bb:cc:dd:ee:ff REACHABLEThe gateway’s IP is the 192.168.1.1, and the MAC is the aa:bb:cc:dd:ee:ff. The REACHABLE is the state, and the STALE is the expired.
The ping. The ping sends the ICMP echo requests and reports the responses.
ping -c 4 192.168.1.1
# PING 192.168.1.1 (192.168.1.1) 56(84) bytes of data.
# 64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.5 ms
# ...
ping -c 4 8.8.8.8
# 64 bytes from 8.8.8.8: icmp_seq=1 ttl=115 time=12.3 msThe first pings the gateway, and the second pings the public address. The gateway’s ping tests the local connectivity, and the public’s ping tests the internet. The two together isolate the problem.
Why the two pings are the diagnosis. The ping to the gateway tests the local network, and the ping to the public address tests the internet. If the gateway’s ping succeeds and the public’s fails, the gateway is up and the internet is down. If the gateway’s ping fails, the local network is the problem.
The traceroute. The traceroute shows the path the packets take.
traceroute 8.8.8.8
# traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
# 1 192.168.1.1 (192.168.1.1) 0.5 ms 0.4 ms 0.5 ms
# 2 10.0.0.1 (10.0.0.1) 2.3 ms 2.2 ms 2.4 ms
# 3 * * *
# 4 8.8.8.8 (8.8.8.8) 12.3 ms 12.2 ms 12.4 msThe output shows the hops from the local to the destination. The * * * is the hop that does not respond, and the 8.8.8.8 is the destination. The traceroute is the path, and the ping is the reachability.
Why the traceroute is the diagnostic. The traceroute shows where the packets stop, which is the failing hop. The ping shows the reachability, and the traceroute shows the path. The two together are the diagnosis.
The mtr. The mtr combines the ping and the traceroute, and it updates the output continuously.
mtr 8.8.8.8The mtr is the modern, and the output is the live. The pattern is the diagnosis, and the use is the continuous.
Why the diagnostics are the layered. The ip route is the table, the ip neigh is the local, the ping is the reachability, and the traceroute is the path. The layers are the diagnosis, and the combination is the complete.
Why the diagnostics should be the systematic. The systematic approach is the interface, the route, the gateway, the DNS, and the destination. The ip addr, the ip route, the ping, and the traceroute are the steps. The systematic is the fast, and the random is the slow.
Complete Example Session
# ============================================
# PART 1: THE ROUTING TABLE
# ============================================
ip route
# default via 192.168.1.1 dev eth0
# 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100
# ============================================
# PART 2: THE ROUTE FOR A DESTINATION
# ============================================
ip route get 8.8.8.8
# 8.8.8.8 via 192.168.1.1 dev eth0 src 192.168.1.100
ip route get 192.168.1.50
# 192.168.1.50 dev eth0 src 192.168.1.100
# ============================================
# PART 3: THE ARP CACHE
# ============================================
ip neigh
# 192.168.1.1 dev eth0 lladdr aa:bb:cc:dd:ee:ff REACHABLE
# ============================================
# PART 4: THE PING
# ============================================
ping -c 4 192.168.1.1
# 64 bytes from 192.168.1.1: icmp_seq=1 ttl=64 time=0.5 ms
# ...
ping -c 4 8.8.8.8
# 64 bytes from 8.8.8.8: icmp_seq=1 ttl=115 time=12.3 ms
# ...
# ============================================
# PART 5: THE TRACEROUTE
# ============================================
traceroute 8.8.8.8
# 1 192.168.1.1 (192.168.1.1) 0.5 ms 0.4 ms 0.5 ms
# 2 10.0.0.1 (10.0.0.1) 2.3 ms 2.2 ms 2.4 ms
# 3 * * *
# 4 8.8.8.8 (8.8.8.8) 12.3 ms
# ============================================
# PART 6: THE STATIC ROUTE
# ============================================
sudo ip route add 10.0.0.0/8 via 192.168.1.2 dev eth0
ip route
# default via 192.168.1.1 dev eth0
# 10.0.0.0/8 via 192.168.1.2 dev eth0
# 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100
# ============================================
# PART 7: THE DEFAULT ROUTE
# ============================================
sudo ip route add default via 192.168.1.1 dev eth0
sudo ip route del default via 192.168.1.1 dev eth0
# ============================================
# PART 8: THE DIAGNOSIS
# ============================================
# The steps:
# 1. ip addr → the interface's address
# 2. ip route → the routing table
# 3. ping the gateway
# 4. ping the public address
# 5. traceroute the public address
# 6. cat /etc/resolv.conf → the DNS
# ============================================
# PART 9: THE SCENARIOS
# ============================================
# The gateway pings, the public does not:
# The gateway is up, the internet is down.
# The DNS is the suspect.
# The gateway does not ping:
# The local network is the problem.
# The interface or the cable is the suspect.
# The public pings, the DNS does not resolve:
# The DNS is the problem.
# The resolv.conf is the suspect.
# ============================================
# PART 10: WHAT NOT TO DO
# ============================================
# Don't add the route without the interface
# sudo ip route add 10.0.0.0/8 via 192.168.1.2 # the interface is guessed
# Don't forget the route's persistence
# The ip route add is temporary.
# Don't assume the gateway is the problem
# The ping is the test.
# Don't forget the DNS
# The ping resolves the name.
# Don't use the wrong subnet for the gateway
# The gateway must be on the local subnet.
# Don't ignore the multiple default routes
# The metric is the tiebreaker.The ten parts cover the routing table, the route for a destination, the ARP cache, the ping, the traceroute, the static route, the default route, the diagnosis, the scenarios, and the anti-patterns.
Quick Reference
The Routing Commands
| Command | Purpose |
|---|---|
ip route | Show the routing table |
ip route get <dest> | Show the route for a destination |
ip route add <dest> via <gw> dev <if> | Add a route |
ip route del <dest> | Delete a route |
ip neigh | Show the ARP cache |
ip addr | Show the addresses |
The Diagnostic Commands
| Command | Purpose |
|---|---|
ping <host> | Test the reachability |
traceroute <host> | Show the path |
mtr <host> | The combined, the live |
dig <name> | Test the DNS |
curl <url> | Test the HTTP |
The Route Types
| Type | Gateway | Interface |
|---|---|---|
| The local subnet | None | The interface |
| The default | The gateway | The interface |
| The static | The gateway | The interface |
| The dynamic | The gateway | The interface |
The ip route Fields
| Field | Meaning |
|---|---|
default | The 0.0.0.0/0 |
via | The gateway |
dev | The interface |
proto | The source (kernel, static, dhcp, ospf) |
scope | The link, the global, the host |
src | The source address |
metric | The preference |
The Diagnosis Steps
| Step | Command |
|---|---|
| 1. The address | ip addr |
| 2. The route | ip route |
| 3. The gateway | ping <gateway> |
| 4. The public | ping 8.8.8.8 |
| 5. The path | traceroute 8.8.8.8 |
| 6. The DNS | dig example.com |
Best Practices
✅ Do This:
# Check the routing table
ip route # ✅
# Check the route for a specific destination
ip route get 8.8.8.8 # ✅
# Test the gateway
ping -c 4 192.168.1.1 # ✅
# Test the public address
ping -c 4 8.8.8.8 # ✅
# Trace the path
traceroute 8.8.8.8 # ✅
# Add the route with the interface
sudo ip route add 10.0.0.0/8 via 192.168.1.2 dev eth0 # ✅
# Check the ARP cache
ip neigh # ✅
# Test the DNS
dig example.com # ✅❌ Don’t Do This:
# Don't add the route without the interface
sudo ip route add 10.0.0.0/8 via 192.168.1.2 # the interface is guessed # ⚠️
# Don't forget the persistence
# The ip route add is temporary. # ⚠️
# Don't assume the gateway is the problem
# The ping is the test. # ⚠️
# Don't forget the DNS
# The ping resolves the name. # ⚠️
# Don't use the wrong subnet for the gateway
# The gateway must be on the local subnet. # ⚠️
# Don't ignore the multiple default routes
# The metric is the tiebreaker. # ⚠️Common Pitfalls
| Pitfall | Problem | Solution |
|---|---|---|
| The missing default route | No internet | Add the default |
| The wrong gateway | The unreachable | Check the subnet |
| The missing interface | The guess | Add dev |
| The temporary route | Lost at reboot | Persist it |
| The DNS failure | The name unresolved | Check resolv.conf |
| The wrong metric | The wrong route | Check the metric |
| The ARP stale | The old MAC | Clear with ip neigh flush |
Real-World Examples
1. The routing table
ip route2. The route for a destination
ip route get 8.8.8.83. The ARP cache
ip neigh4. The gateway ping
ping -c 4 192.168.1.15. The public ping
ping -c 4 8.8.8.86. The traceroute
traceroute 8.8.8.87. The static route
sudo ip route add 10.0.0.0/8 via 192.168.1.2 dev eth08. The default route
sudo ip route add default via 192.168.1.1 dev eth09. The DNS test
dig example.com10. The combined
mtr 8.8.8.8Visual: The Gateway
┌──────────────────────────────────────────────────────────┐
│ THE LOCAL SUBNET 192.168.1.0/24 │
│ │
│ The device: 192.168.1.100 │
│ The gateway: 192.168.1.1 │
│ │
│ THE DEVICE SENDS THE LOCAL PACKET: │
│ The destination is 192.168.1.50. │
│ The ARP resolves the MAC. │
│ The packet is sent to the MAC. │
│ │
│ THE DEVICE SENDS THE REMOTE PACKET: │
│ The destination is 8.8.8.8. │
│ The ARP resolves the gateway's MAC. │
│ The packet is sent to the gateway. │
│ The gateway forwards the packet. │
│ │
└──────────────────────────────────────────────────────────┘Visual: The Routing Table
┌──────────────────────────────────────────────────────────┐
│ ip route │
│ │
│ default via 192.168.1.1 dev eth0 │
│ │ │ │ │
│ │ │ └── the interface │
│ │ └── the gateway │
│ └── the destination (0.0.0.0/0) │
│ │
│ 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100│
│ │ │ │ │ │ │ │
│ │ │ │ │ │ └── the source │
│ │ │ │ │ └── the link scope │
│ │ │ │ └── the kernel added │
│ │ │ └── the interface │
│ │ └── no gateway (the direct) │
│ └── the destination │
│ │
└──────────────────────────────────────────────────────────┘Visual: The Routing Decision
┌──────────────────────────────────────────────────────────┐
│ THE DESTINATION 8.8.8.8 │
│ │ │
│ ▼ │
│ The kernel checks the routing table. │
│ │ │
│ ├── The 192.168.1.0/24? The 8.8.8.8 does not match.│
│ │ │
│ └── The default? The 0.0.0.0/0 matches. │
│ │ │
│ ▼ │
│ The route: default via 192.168.1.1 dev eth0. │
│ │ │
│ ▼ │
│ The packet is sent to the gateway. │
│ │
│ THE DESTINATION 192.168.1.50 │
│ │ │
│ ▼ │
│ The 192.168.1.0/24 matches. │
│ The route: dev eth0 (the direct). │
│ The packet is sent to the destination's MAC. │
│ │
└──────────────────────────────────────────────────────────┘Visual: The Diagnosis
┌──────────────────────────────────────────────────────────┐
│ 1. ip addr → the interface's address │
│ Is the address correct? │
│ │
│ 2. ip route → the routing table │
│ Is the default route present? │
│ │
│ 3. ping the gateway │
│ Is the local network working? │
│ │
│ 4. ping the public address │
│ Is the internet working? │
│ │
│ 5. traceroute the public address │
│ Where does the path stop? │
│ │
│ 6. dig example.com │
│ Is the DNS resolving? │
│ │
│ The systematic approach isolates the failure. │
│ │
└──────────────────────────────────────────────────────────┘Visual: The Static Route
┌──────────────────────────────────────────────────────────┐
│ sudo ip route add 10.0.0.0/8 via 192.168.1.2 dev eth0 │
│ │ │ │ │ │
│ │ │ │ └── the interface│
│ │ │ └── the gateway │
│ │ └── the destination │
│ └── the command │
│ │
│ ip route │
│ default via 192.168.1.1 dev eth0 │
│ 10.0.0.0/8 via 192.168.1.2 dev eth0 │
│ 192.168.1.0/24 dev eth0 proto kernel scope link src 192.168.1.100│
│ │
│ The 10.0.0.0/8 packets go to the 192.168.1.2 gateway. │
│ │
└──────────────────────────────────────────────────────────┘Summary
| Item | Value |
|---|---|
| Gateway | The router on the local subnet |
| Default route | The 0.0.0.0/0 |
| Routing table | The list of the routes |
| The decision | The longest-prefix match |
| The direct route | No gateway |
| The gateway route | The via |
| The static route | The manual |
| The dynamic route | The protocol |
| Command | Purpose |
|---|---|
ip route | The table |
ip route get <dest> | The route for a destination |
ip neigh | The ARP cache |
ping <host> | The reachability |
traceroute <host> | The path |
mtr <host> | The combined |
Key takeaways:
- The gateway is the router that connects the local subnet to the other networks — every packet whose destination is not on the local subnet is sent to the gateway
- The default route is the
0.0.0.0/0— it matches every destination and is the last resort when no more specific route matches - The routing table is the list of the routes — each has the destination, the gateway, the interface, the protocol, the scope, the source, and the metric
- The routing decision is the longest-prefix match — the most specific route wins, and the default is the fallback
- The local subnet route is the direct — no gateway, and the packet is sent to the destination’s MAC
- The gateway route has the
via— the packet is sent to the gateway, and the gateway forwards it - The static route is the manual, and the dynamic is the learned — the static is the small network, and the dynamic is the large
- The
ip route getshows the route for a specific destination — it is the way to verify the routing decision - The diagnosis is the systematic — the address, the route, the gateway, the public, the path, and the DNS
- The
pingtests the reachability, and thetracerouteshows the path — the two together isolate the failure
Remember: The gateway is the device’s link to the outside, and the routing table is the kernel’s decision. The default route is the last resort, the longest prefix is the rule, and the ip route is the table. The diagnosis is the layered — the address, the route, the gateway, the public, the path, and the DNS — and the systematic approach is the fast. The gateway is the entry, and the routing is the path.
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