Local traffic delivery

The local delivery problem

An IP address is not the whole local delivery story.

When a destination is inside the same subnet, a device can deliver the frame directly on the local network. It still needs the local hardware address that should receive the frame.

ARP provides that bridge for IPv4: a device asks which local hardware address owns an IP address, then stores the answer briefly in a cache.

Local delivery and routing answer different questions.Local delivery finds the next interface on the current network. Routing decides whether the destination is local or needs another network.

Address resolution

ARP and RARP resolve addresses in opposite directions.

01

ARP

Starts with an IPv4 address and discovers the local MAC address needed for the Ethernet frame.

02

RARP

Starts with a device MAC address and asks for an IPv4 address, mainly for older diskless systems.

RARP is historical.RARP was limited and has largely been replaced by BOOTP and DHCP, which can assign an address and provide configuration such as a gateway and DNS server.

Build the model

Four delivery ideas

01

Unicast

One sender delivers to one selected destination.

02

Broadcast

One sender delivers to every device in the local broadcast domain.

03

Multicast

One sender delivers to devices that joined a selected group.

04

ARP

Maps an IPv4 address to a local hardware address so a frame can be delivered.

Mini experiment

Choose the destination pattern

Select a delivery goal and see who should receive the frame.

PATTERNUnicast

RECEIVERSOne selected device

WHYUse the destination hardware address discovered for the local IP.

Unicast keeps local delivery focused on one destination.

Knowledge check

Test your local delivery model

Answer six questions. The answer order changes each time.

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