The local delivery problem
An IP address is not the whole local delivery story.
On an Ethernet network, a device needs the next interface's MAC (Media Access Control) address to deliver a frame. For an on-link destination, that interface belongs to the destination; for an off-link destination, it normally belongs to the next-hop router.
ARP resolves an IPv4 address to a local MAC address. A request is normally broadcast and the reply is normally unicast. A usable cached mapping avoids repeating the request for every packet.
IPv6 uses Neighbor Discovery rather than ARP. ARP does not authenticate replies: a received mapping alone is not proof of a device's identity.
Address resolution
ARP and RARP resolve addresses in opposite directions.
ARP
Starts with an IPv4 address and discovers the local MAC address needed for the Ethernet frame.
RARP
RARP (Reverse Address Resolution Protocol) starts with a device MAC address and asks for an IPv4 address, mainly for older diskless systems.
BOOTP means Bootstrap Protocol; DHCP means Dynamic Host Configuration Protocol; DNS means Domain Name System.
Build the model
Four delivery ideas
Unicast
One sender delivers to one selected destination.
Broadcast
One sender delivers to every device in the local broadcast domain.
Multicast
One sender delivers to devices that joined a selected group.
ARP
Maps an IPv4 address to a local hardware address so a frame can be delivered.
Local Ethernet broadcasts normally stay within their broadcast domain; routers do not forward them to every network. Receiving a frame does not mean every application accepts its contents.
Multicast describes intended group receivers, not a guarantee that only their switch ports receive the frame. A switch may flood multicast when group forwarding state is missing.
Mini experiment
Choose the destination pattern
Select a delivery goal and compare its intended receivers. This simplified model does not show every port a switch may forward to.
Knowledge check