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Tejas Shinkar
Tejas Shinkar

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Routers ā€” How ARP and Routing Work Together

#networking #aws #cloud #devops

šŸŒ Routers ā€” Part 2

How ARP + Routing Work Together

Series: Networking Fundamentals for Cloud & DevOps

This part mainly explains how ARP + routing work together when Host A sends data to Host C through R1 and R2.

Before following the journey, keep these two tables separate:

Table Layer Main Question
Routing Table L3 Where should I send the packet next?
ARP Table L2 What MAC address do I need for the next hop?

So:

Routing Table:
Destination IP/network ā†’ where to send next

ARP Table:
IP address ā†’ MAC address
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This distinction is extremely important.

Example: A ā†’ C

Consider this network:

       10.0.44.0/24        10.0.55.0/24        10.0.66.0/24

A                         R1                    R2                    C
10.0.44.9                 .1                    .2                 10.0.66.7
MAC a9a9                  eee1                  eee2                  c7c7

       ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”¬ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”¬ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€
                                ā”‚                       ā”‚
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More simply:

A ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ R1 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ R2 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ C
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With:

A:
IP  = 10.0.44.9
MAC = a9a9

R1:
IP  = 10.0.44.1
MAC = eee1

R2:
IP  = 10.0.55.2
MAC = eee2

C:
IP  = 10.0.66.7
MAC = c7c7
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A wants to send data to C.

The destination is:

10.0.66.7
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Step 1 ā€” A Creates the L3 Packet

The destination IP remains C's IP:

SRC IP = 10.0.44.9
DST IP = 10.0.66.7
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A checks its routing information and realizes:

10.0.66.7 is outside my local network.
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So A needs to send the packet to its default gateway, R1.

The important point is:

The final destination IP is C, but the next hop is R1.

Step 2 ā€” A Needs R1's MAC

A knows R1's IP:

R1 IP = 10.0.44.1
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But A doesn't know R1's MAC yet:

10.0.44.1 ā†’ ??? MAC
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So A sends an ARP Request:

"Who has 10.0.44.1?"
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R1 replies:

10.0.44.1 ā†’ eee1
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A stores this mapping in its ARP table.

ARP Table

10.0.44.1 ā†’ eee1
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Now A knows the Layer 2 destination needed for the first hop.

Step 3 ā€” A Sends the Frame to R1

A can now construct the Layer 2 frame.

L2 

SRC MAC = a9a9
DST MAC = eee1
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L3 

SRC IP = 10.0.44.9
DST IP = 10.0.66.7
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Notice the important thing:

MAC destination = R1

IP destination = C

So the frame goes:

A ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ R1
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while the IP packet still says:

10.0.44.9 ā†’ 10.0.66.7
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Step 4 ā€” R1 Receives It

R1 receives the frame because:

DST MAC = eee1
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which is R1's MAC address.

R1 removes/discards the old Layer 2 header.

The IP information is still:

SRC IP = 10.0.44.9
DST IP = 10.0.66.7
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R1 checks its routing table:

10.0.66.0/24 ā†’ 10.0.55.2
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The meaning is:

To reach C's network, send the packet to R2 at 10.0.55.2.

This is the Layer 3 routing decision.

Step 5 ā€” R1 Needs R2's MAC

R1 knows R2's IP:

10.0.55.2
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but it needs R2's MAC:

10.0.55.2 ā†’ ??? MAC
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So R1 sends ARP:

"Who has 10.0.55.2?"
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R2 replies:

10.0.55.2 ā†’ eee2
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R1 stores:

ARP Table

10.0.55.2 ā†’ eee2
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Now R1 knows the Layer 2 destination for the next hop.

Step 6 ā€” R1 Sends a NEW Layer 2 Frame

This is the most important concept in this entire example.

R1 creates a new Layer 2 header:

L2:

SRC MAC = eee1
DST MAC = eee2
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But the Layer 3 addresses remain:

L3:

SRC IP = 10.0.44.9
DST IP = 10.0.66.7
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Notice:

MAC addresses ā†’ CHANGED

10.0.44.9 ā†’ 10.0.66.7
IP addresses ā†’ STAY THE SAME
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The frame is now:

R1 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ R2
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The IP packet is still ultimately going to C.

Step 7 ā€” R2 Receives It

R2 receives the frame because:

DST MAC = eee2
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which is R2's MAC address.

R2 removes the Layer 2 header and checks the destination IP:

DST IP = 10.0.66.7
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R2's routing table says:

10.0.66.0/24 ā†’ Left interface
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C is directly connected to R2.

So R2 now needs C's MAC address.

It sends ARP:

"Who has 10.0.66.7?"
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C replies:

10.0.66.7 ā†’ c7c7
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R2 stores:

ARP Table

10.0.66.7 ā†’ c7c7
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Step 8 ā€” R2 Sends the Frame to C

R2 creates another new Layer 2 header:

L2:

SRC MAC = eee4
DST MAC = c7c7
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The Layer 3 information remains:

L3:

SRC IP = 10.0.44.9
DST IP = 10.0.66.7
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So:

R2 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ C
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C receives the frame because:

DST MAC = c7c7
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C checks the destination IP:

DST IP = 10.0.66.7
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That's C's own IP.

Therefore:

C accepts the packet.

The Entire Journey

The complete journey can be visualized as:

A                         R1                         R2                         C
ā”‚                          ā”‚                          ā”‚                          ā”‚
ā”‚ā”€ā”€ ARP R1 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ā”‚                          ā”‚                          ā”‚
ā”‚ā†ā”€ 10.0.44.1 / eee1 ā”€ā”€ā”€ā”€ā”‚                          ā”‚                          ā”‚
ā”‚                          ā”‚                          ā”‚                          ā”‚
ā”‚ā”€ā”€ a9a9 ā†’ eee1 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ā”‚                          ā”‚                          ā”‚
ā”‚      IP: A ā†’ C           ā”‚                          ā”‚                          ā”‚
ā”‚                          ā”‚                          ā”‚                          ā”‚
ā”‚                          ā”‚ā”€ā”€ ARP R2 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ā”‚                          ā”‚
ā”‚                          ā”‚ā†ā”€ 10.0.55.2 / eee2 ā”€ā”€ā”€ā”€ā”€ā”‚                          ā”‚
ā”‚                          ā”‚                          ā”‚                          ā”‚
ā”‚                          ā”‚ā”€ā”€ eee1 ā†’ eee2 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ā”‚                          ā”‚
ā”‚                          ā”‚      IP: A ā†’ C           ā”‚                          ā”‚
ā”‚                          ā”‚                          ā”‚                          ā”‚
ā”‚                          ā”‚                          ā”‚ā”€ā”€ ARP C ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ā”‚
ā”‚                          ā”‚                          ā”‚ā†ā”€ 10.0.66.7 / c7c7 ā”€ā”€ā”€ā”€ā”‚
ā”‚                          ā”‚                          ā”‚                          ā”‚
ā”‚                          ā”‚                          ā”‚ā”€ā”€ eee4 ā†’ c7c7 ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā”€ā†’ā”‚
ā”‚                          ā”‚                          ā”‚      IP: A ā†’ C           ā”‚
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The key pattern is:

At every router hop:

L2 / MAC header ā†’ CHANGES
L3 / IP header   ā†’ STAYS THE SAME
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Routing Table vs ARP Table

This is one of the most important distinctions to remember.

Routing Table

Answers:

"Where should I send this packet next?"

Example:

10.0.66.0/24 ā†’ 10.0.55.2
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The router uses this information to select the next hop/interface.

ARP Table

Answers:

"What MAC address do I need for that next hop?"

Example:

10.0.55.2 ā†’ eee2
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So the relationship is:

Destination network
        ā†“
Routing Table
        ā†“
Next-hop IP
        ā†“
ARP Table
        ā†“
Next-hop MAC
        ā†“
Build Layer 2 frame
        ā†“
Send
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The One Thing to Remember

At every router hop:

L2 / MAC header ā†’ CHANGES
L3 / IP header   ā†’ STAYS THE SAME
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For this example:

Hop Source MAC Destination MAC Source IP Destination IP
A ā†’ R1 a9a9 eee1 10.0.44.9 10.0.66.7
R1 ā†’ R2 eee1 eee2 10.0.44.9 10.0.66.7
R2 ā†’ C eee4 c7c7 10.0.44.9 10.0.66.7

This table captures the whole concept:

Routing table tells the router WHERE to send the packet.

ARP table tells the router WHICH MAC address is needed for the next hop.

Final Mental Model 

ROUTER
ā”‚
ā”œā”€ā”€ Routing Table
ā”‚     ā””ā”€ā”€ Destination network ā†’ Next hop / Interface
ā”‚
ā”œā”€ā”€ ARP Table
ā”‚     ā””ā”€ā”€ Next-hop IP ā†’ MAC address
ā”‚
ā””ā”€ā”€ Forwarding process
      ā”‚
      ā”œā”€ā”€ Check destination IP
      ā”‚
      ā”œā”€ā”€ Routing table decides next hop
      ā”‚
      ā”œā”€ā”€ ARP resolves next-hop MAC
      ā”‚
      ā”œā”€ā”€ Build a NEW L2 frame
      ā”‚
      ā””ā”€ā”€ Forward to next hop
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And the simplest possible memory rule is:

Routing Table
ā†’ Where should the packet go?

ARP Table
ā†’ What MAC gets me to the next hop?

IP
ā†’ Final destination

MAC
ā†’ Current hop
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This is the connection between everything we've learned so far:

IP + Routing
      ā†“
Which network should the packet reach?

ARP
      ā†“
Which MAC address gets the frame to the next hop?

Ethernet / Layer 2
      ā†“
Move the frame across the current hop
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For Cloud + DevOps, this mental model becomes especially useful when understanding VPC route tables, subnets, gateways, next hops, and connectivity troubleshooting.

Part 5 of 6 ā€” Networking Fundamentals for Cloud & DevOps

Next: deeper router behavior and how routing decisions are made.

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