Tejas Shinkar

Posted on Aug 18 Edited on Aug 19

Routers — How ARP and Routing Work Together

#devops #cloud #networking #aws

🌐 Routers — Part 2

How ARP + Routing Work Together

Series: Networking Fundamentals for Cloud & DevOps — Part 5 of 6

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?

Routing Table: destination IP/network → where to send next.
ARP Table: IP address → MAC address.

This distinction is extremely important.

Example: A → 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

       ────────────────────────┬───────────────────────┬──────────────────────
                                │                       │

More simply: A ── R1 ── R2 ── C, 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

A wants to send data to C, at destination 10.0.66.7.

Step 1 — A Creates the L3 Packet

The destination IP remains C's IP: SRC IP = 10.0.44.9, DST IP = 10.0.66.7.

A checks its routing information and realizes 10.0.66.7 is outside its local network. 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 — A Needs R1's MAC

A knows R1's IP (10.0.44.1) but doesn't know R1's MAC yet. So A sends an ARP Request:

"Who has 10.0.44.1?"

R1 replies 10.0.44.1 → eee1, and A stores this mapping in its ARP table (10.0.44.1 → eee1). Now A knows the Layer 2 destination needed for the first hop.

Step 3 — A Sends the Frame to R1

A can now construct the Layer 2 frame:

L2:  SRC MAC = a9a9         DST MAC = eee1
L3:  SRC IP  = 10.0.44.9    DST IP  = 10.0.66.7

Notice the important thing:

MAC destination = R1

IP destination = C

So the frame goes A ─────────────────→ R1, while the IP packet still says 10.0.44.9 → 10.0.66.7.

Step 4 — R1 Receives It

R1 receives the frame because DST MAC = eee1, 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.

R1 checks its routing table: 10.0.66.0/24 → 10.0.55.2.

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

This is the Layer 3 routing decision.

Step 5 — R1 Needs R2's MAC

R1 knows R2's IP (10.0.55.2) but needs R2's MAC. So R1 sends ARP:

"Who has 10.0.55.2?"

R2 replies 10.0.55.2 → eee2. R1 stores this in its ARP table (10.0.55.2 → eee2). Now R1 knows the Layer 2 destination for the next hop.

Step 6 — R1 Sends a NEW Layer 2 Frame

This is the most important concept in this entire example. R1 creates a new Layer 2 header, while the Layer 3 addresses remain unchanged:

L2:  SRC MAC = eee1         DST MAC = eee2
L3:  SRC IP  = 10.0.44.9    DST IP  = 10.0.66.7   (unchanged)

MAC addresses → changed. IP addresses (10.0.44.9 → 10.0.66.7) → stay the same.

The frame is now R1 ─────────────────→ R2. The IP packet is still ultimately going to C.

Step 7 — R2 Receives It

R2 receives the frame because DST MAC = eee2, which is R2's MAC address. R2 removes the Layer 2 header and checks the destination IP (DST IP = 10.0.66.7).

R2's routing table says 10.0.66.0/24 → Left interface — C is directly connected to R2. So R2 now needs C's MAC address. It sends ARP:

"Who has 10.0.66.7?"

C replies 10.0.66.7 → c7c7. R2 stores this in its ARP table (10.0.66.7 → c7c7).

Step 8 — R2 Sends the Frame to C

R2 creates another new Layer 2 header, while the Layer 3 information remains the same:

L2:  SRC MAC = eee4         DST MAC = c7c7
L3:  SRC IP  = 10.0.44.9    DST IP  = 10.0.66.7

So R2 ─────────────────→ C. C receives the frame because DST MAC = c7c7, and checks the destination IP (DST IP = 10.0.66.7) — that's C's own IP.

C accepts the packet.

The Entire Journey

The complete journey can be visualized as:

A                         R1                         R2                         C
│                          │                          │                          │
│── ARP R1 ──────────────→│                          │                          │
│←─ 10.0.44.1 / eee1 ────│                          │                          │
│                          │                          │                          │
│── a9a9 → eee1 ─────────→│                          │                          │
│      IP: A → C           │                          │                          │
│                          │                          │                          │
│                          │── ARP R2 ──────────────→│                          │
│                          │←─ 10.0.55.2 / eee2 ─────│                          │
│                          │                          │                          │
│                          │── eee1 → eee2 ─────────→│                          │
│                          │      IP: A → C           │                          │
│                          │                          │                          │
│                          │                          │── ARP C ───────────────→│
│                          │                          │←─ 10.0.66.7 / c7c7 ────│
│                          │                          │                          │
│                          │                          │── eee4 → c7c7 ─────────→│
│                          │                          │      IP: A → C           │

At every router hop: L2/MAC header → changes. L3/IP header → stays the same.

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?" — e.g. 10.0.66.0/24 → 10.0.55.2. The router uses this information to select the next hop/interface.

ARP Table answers: "What MAC address do I need for that next hop?" — e.g. 10.0.55.2 → eee2.

So the relationship is:

Destination network
        ↓
Routing Table
        ↓
Next-hop IP
        ↓
ARP Table
        ↓
Next-hop MAC
        ↓
Build Layer 2 frame
        ↓
Send

The One Thing to Remember

At every router hop: L2/MAC header → changes. L3/IP header → stays the same.

For this example:

Hop	Source MAC	Destination MAC	Source IP	Destination IP
A → R1	a9a9	eee1	10.0.44.9	10.0.66.7
R1 → R2	eee1	eee2	10.0.44.9	10.0.66.7
R2 → 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
│
├── Routing Table
│     └── Destination network → Next hop / Interface
│
├── ARP Table
│     └── Next-hop IP → MAC address
│
└── Forwarding process
      │
      ├── Check destination IP
      │
      ├── Routing table decides next hop
      │
      ├── ARP resolves next-hop MAC
      │
      ├── Build a NEW L2 frame
      │
      └── Forward to next hop

And the simplest possible memory rule:

Routing Table → Where should the packet go?
ARP Table → What MAC gets me to the next hop?
IP → Final destination
MAC → Current hop

This is the connection between everything we've learned so far:

IP + Routing → which network should the packet reach? ARP → which MAC address gets the frame to the next hop? Ethernet / Layer 2 → move the frame across the current hop.

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 — Networking Fundamentals for Cloud & DevOps
Next: deeper router behavior and how routing decisions are made.

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