The supporting services 13 min read
ARP, DHCP, DNS, and Network Time
Connect names and IP addresses to Ethernet delivery, see how automatic addressing works, and separate ordinary clock setting from precision media synchronization.
- Explain how ARP connects IPv4 addresses to local MAC addresses.
- Follow the basic DHCP address-assignment exchange.
- Distinguish DNS name resolution from discovery protocols.
- Separate NTP wall-clock time from PTP media clock synchronization.
ARP completes local delivery
An endpoint may know the local destination's IP address but still needs a destination MAC address for the Ethernet frame. ARP asks the local network which interface owns that IPv4 address. The owner replies, and the result is temporarily cached.
For a remote destination, the sender does not ARP for the remote host. It ARPs for its default gateway, because that is the next Ethernet destination on the local network.
DHCP supplies a usable configuration
A new DHCP client begins without a usable IPv4 configuration. The familiar sequence is Discover, Offer, Request, and Acknowledge—often shortened to DORA. The resulting lease can include an IP address, subnet mask, gateway, DNS servers, lease duration, and other options.
Because the initial client does not yet know its network, DHCP begins with broadcasts. A DHCP relay can forward those requests to a server in another subnet. If the wrong DHCP server answers, devices may receive perfectly valid information for the wrong network.
DNS translates names; it does not find everything
DNS translates names such as av-core.example.com into IP addresses and can provide other records. It is a hierarchical service that may operate well beyond the local VLAN.
Product discovery is often different. Dante, AirPlay, printers, and other platforms may use multicast DNS, proprietary broadcasts, or dedicated discovery services. Successful DNS resolution therefore does not prove that an AV management application will discover its endpoints across VLANs.
NTP and PTP solve different timing problems
NTP keeps computers, logs, certificates, schedules, and control systems close to ordinary clock time. PTP distributes much more precise timing for synchronized media clocks.
Native Dante commonly uses PTPv1, while AES67, Q-LAN, and many broadcast systems use PTPv2 profiles. PTP version and PTP domain number are separate concepts. Multiple PTP versions or numbered domains can coexist on a VLAN, but devices synchronize only with a compatible version, profile, and domain.
Put it in an AV system.
Wrong DHCP scope
An encoder links correctly but receives a corporate address rather than an AV address. Inspect the access VLAN, DHCP relay, and competing DHCP servers before changing the encoder.
Clocked but undiscovered
AES67 devices may share PTP timing while a controller still cannot discover or route them. Clock synchronization, discovery, and control are separate services.
- ARP
- Address Resolution Protocol; maps a local IPv4 address to a MAC address.
- DHCP
- Dynamic Host Configuration Protocol; leases IP configuration to clients.
- DNS
- Domain Name System; resolves names and related records.
- NTP
- Network Time Protocol; ordinary network clock synchronization.
- PTP
- Precision Time Protocol; high-accuracy clock synchronization used by time-sensitive systems.
Inspect the supporting evidence
On a computer connected to an AV network, record its DHCP-provided address, mask, gateway, and DNS server. Inspect the ARP or neighbor table, then identify the NTP server and any PTP leader shown by the AV platform.
Three quick questions.
01For a remote IP destination, whose MAC address does the sender need first?
The local default gateway's MAC address.
02What information can DHCP provide besides an IP address?
Common options include the subnet mask, gateway, DNS servers, and lease duration.
03Are PTPv1, PTPv2, and PTP domain 2 the same kind of label?
No. v1 and v2 are protocol versions; the domain number identifies a logical clock community within a compatible PTP implementation.