← ACARA Digital Technologies Years 9–10

Routing table and next hop

A list a router uses to pick where to send each packet. Each row has a destination network and a next hop. The default route 0.0.0.0 is used when no other row matches.

Part of: ACARA Digital Technologies Years 9–10 — Networked Systems and Access Control — Module 41, WA Computer Science ATAR Year 11 — Network Communications — Module 68

Understanding Routing table and next hop in ACARA Digital Technologies Years 9–10

What it does: A /24 row names a smaller, more exact network than a /8 row, so it is the best match for addresses inside it. The default route is the catch-all row, often pointing to the ISP router, used when no other destination network matches. The next hop is only the next step on the journey. The packet may pass through several more routers before it arrives. The default route is the last resort. More specific rows are checked first. A routing table is a list of directions for choosing where to send packets. It does not store the data itself.

Common mistakes: The default route is 0.0.0.0/0, which matches every destination. 192.168.0.0/24 is just one ordinary network, so it can't act as the catch-all route. The default route destination must be 0.0.0.0/0. 255.255.255.255/0 is not the default route, so the router would have no catch-all row.

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This module's key terms are also in a printable crossword (Modules 41–55) and word search (Modules 12–55).

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Learning

Duo

What does each row in a routing table tell a router?

Which password a device must use to join the networkWhich destination network to match and which next hop to send those packets to

Matches

Routing table row: 192.168.5.0/24 via 192.168.5.77 (a device on that network)

↓

The next hop must be a router that forwards the packet, not the final device the packet is for.

Applying

Meanings

Term

routing table

Meaning

A list a router uses to pick where to send each packet. Each row has a destination network and a next hop. The default route 0.0.0.0 is used when no other row matches.

Practice

destination next_hop

Testing

Gaps

via 192.168.1.1

Check Answer

Spots

1Default route: 192.168.0.0/24

2Used when no other row matches

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Quick Questions

1. What does each row in a routing table tell a router?

A. Which destination network to match and which next hop to send those packets to ✓

B. Which password a device must use to join the network

A row pairs a destination network with a next hop, which is the next router to pass the packet to.

2. What is the 'next hop' in a routing table row?

A. The next router the packet is handed to on its way ✓

B. The final computer that receives the packet

The next hop is only the next step on the journey. The packet may pass through several more routers before it arrives.

3. A router has these rows: 192.168.1.0 via 10.0.0.2 and 0.0.0.0 via 10.0.0.9. A packet is addressed to 8.8.8.8. Which next hop is used?

A. 10.0.0.2, because it is the first row in the table

B. 10.0.0.9, because the default route is used when nothing else matches ✓

8.8.8.8 does not match 192.168.1.0, so the router falls back to the default route 0.0.0.0.

4. When is the default route 0.0.0.0 used?

A. Every time, before the router checks any other row

B. When no other row in the table matches the destination ✓

The default route is the last resort. More specific rows are checked first.

5. Two rows match a packet's destination. One is a small, specific network and the other is the default route. Which does the router choose?

A. The more specific network, because it is the best match ✓

B. The default route, because it covers every address

The router picks the best (most specific) match. The default route only wins when nothing better matches.

Match the Pairs

192.168.2.0/24 via 10.0.0.2 → A routing table row: packets for the 192.168.2.0/24 network are sent to the next hop 10.0.0.2.

0.0.0.0/0 via 10.0.0.1 → The default route: packets that match no other row go to the next hop 10.0.0.1.

A router picks the most specific row that matches the destination, so a /24 row beats a /8 row. → 10.1.2.0/24 via 10.0.0.3

The default route sends all unmatched traffic, such as packets bound for the internet, to one next hop. → 0.0.0.0/0 via 203.0.113.1

Routing table row: 192.168.5.0/24 via 192.168.5.77 (a device on that network) → The next hop must be a router that forwards the packet, not the final device the packet is for.

Packet for 8.8.8.8 matches a 10.0.0.0/8 row, so the router uses that row instead of the default route 0.0.0.0 → 8.8.8.8 is not inside 10.0.0.0/8, so that row does not match and the default route is used.

Key Term

routing table

A list a router uses to pick where to send each packet. Each row has a destination network and a next hop. The default route 0.0.0.0 is used when no other row matches.

Fill in the Blanks

1. 0.0.0.0/0 via 192.168.1.1

Hint: Type the destination that stands for the default route and matches every address.

2. ip route add 10.1.0.0/16 via 10.0.0.2

Hint: Type the keyword that comes before the next hop address.

3. 10.1.0.0/16 via 10.0.0.2 10.1.5.0/24 via 10.0.0.3 Packet to 10.1.5.9 -> 10.0.0.3

Hint: Type the next hop from the row that matches the packet most specifically.

4. 10.1.0.0/16 via 10.0.0.2 0.0.0.0/0 via 10.0.0.1 8.8.8.8 matches no row, so use default route

Hint: Type the word for the route used when no other row matches.

5. ip route show default via 192.168.1.1 dev wlan0

Hint: Type the word for the table of destinations and next hops that the command displays.

Spot the Error

Question 1

A Default route: 192.168.0.0/24

B Used when no other row matches

C 0.0.0.0/0 -> 10.1.1.1

Line A — The default route is 0.0.0.0/0, which matches every destination. 192.168.0.0/24 is just one ordinary network, so it can't act as the catch-all route.

Question 2

A # destination -> next hop

B 192.168.1.0/24 -> direct

C 10.0.0.0/8 -> 10.1.1.2

D 255.255.255.255/0 -> 10.1.1.1

Line D — The default route destination must be 0.0.0.0/0. 255.255.255.255/0 is not the default route, so the router would have no catch-all row.

Question 3

A dest = 8.8.8.8

B if no row matches dest:

C drop the packet

D else:

E use the matching row

Line C — When no other row matches, the router uses the default route and sends the packet to the default next hop. It should not drop the packet.

Question 4

A 10.0.0.0/8 -> 10.1.1.2

B 10.5.0.0/16 -> 10.1.1.3

C Packet to 10.5.2.9 uses row 1

D Longer prefix = best match

Line C — 10.5.2.9 matches both rows, but row 2 (/16) has the longer prefix, so it is the best match. The packet should use row 2, not row 1.

Question 5

A dest = 172.16.4.2

B row 1: 10.0.0.0/8 no match

C row 2: 172.16.0.0/12 matches

D send to next hop of row 1

Line D — Row 1 did not match the destination. The packet must go to the next hop of the matching row, which is row 2.

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