Port 547: DHCPv6 server — Where IPv6 addresses get their assignments

Port 547 carries DHCPv6 server traffic—the IPv6 version of automatic network configuration. When a device joins an IPv6 network and needs an address, DNS servers, or other network settings from a central authority, this is where that conversation happens.

What Runs on Port 547

Protocol: DHCPv6 (Dynamic Host Configuration Protocol for IPv6)
Transport: UDP
Port pair: Servers listen on 547, clients listen on 546

DHCPv6 servers and relay agents listen on port 547 for configuration requests from IPv6 clients. This is the stateful address assignment system for IPv6—where a server keeps track of which device got which address, when the lease expires, and what additional configuration was provided.¹

How DHCPv6 Works

A DHCPv6 exchange follows this pattern:

1. Client discovers: Device sends a SOLICIT message from port 546 to port 547, often as multicast to find available DHCPv6 servers
2. Server advertises: DHCPv6 server responds with available addresses and configuration
3. Client requests: Device asks for a specific address or confirms the offered configuration
4. Server assigns: Server confirms the assignment and records it in its lease database

The server remembers this. That's the difference. Port 547 traffic represents stateful configuration—every assignment tracked, every lease timed, every renewal logged.²

The History: Why IPv6 Needed Its Own DHCP

DHCP for IPv4 was defined in RFC 1531 in 1993, using ports 67 and 68. When IPv6 arrived, it couldn't just reuse those ports. IPv6 addresses are 128 bits instead of 32. The packet structure changed. The entire addressing philosophy shifted.

DHCPv6 was formally defined in RFC 3315 in July 2003—a decade after IPv4 DHCP.³ It needed its own ports: 546 for clients, 547 for servers. The protocol was designed alongside SLAAC (Stateless Address Autoconfiguration), giving IPv6 networks a choice that IPv4 never really had.

SLAAC vs. DHCPv6: The Choice IPv6 Offers

Here's what makes IPv6 different: you don't have to use DHCPv6 at all.

SLAAC (Stateless Address Autoconfiguration) lets devices generate their own IPv6 addresses using router advertisements and their MAC address. No server. No tracking. No port 547 traffic. The device just figures it out.⁴

Stateful DHCPv6 (port 547) means a server assigns and tracks every address. The server knows who you are, what address you got, when your lease expires. This is what most IPv4 networks use, and what many IPv6 networks choose when they want central control.⁵

Stateless DHCPv6 is the middle ground—use SLAAC for the address itself, but ask a DHCPv6 server (still port 547) for DNS servers, domain names, and other options. Get an address on your own, but get directions from someone who knows the network.⁶

The router advertisement tells clients which mode to use through flags. M flag set to 1? Use stateful DHCPv6. O flag set to 1 with M flag at 0? Use SLAAC but grab DNS info from DHCPv6. Both flags at 0? Pure SLAAC, no port 547 needed.

Why Two Ports?

IPv4 DHCP uses port 67 for servers and port 68 for clients. DHCPv6 follows the same pattern: port 547 for servers and relay agents, port 546 for clients.

Why separate ports? Because DHCP operates before the client has a permanent address. The client needs a known port to listen on (546) while it sends requests to a known server port (547). When the server replies, it sends to the client's port. The separation makes the bidirectional conversation work when one party doesn't have an address yet.⁷

You need an address to speak, but to speak you need an address. The port separation is how the protocol solves that paradox.

Security Considerations

Port 547 should be carefully firewalled:

• Restrict to local networks: DHCPv6 servers shouldn't be reachable from the Internet
• Rogue server risk: An attacker running a fake DHCPv6 server on port 547 can redirect traffic, provide malicious DNS servers, or intercept communications
• Relay agent security: DHCPv6 relay agents forward requests between network segments—ensure they're configured to prevent abuse
• Lease exhaustion attacks: An attacker can request all available addresses, denying service to legitimate clients

Some networks disable DHCPv6 entirely and rely on SLAAC with router advertisements for DNS configuration (RDNSS). Others require DHCPv6 for the control and visibility it provides.

Related Ports

Port 546 — DHCPv6 client port, where devices listen for server responses
Port 67 — IPv4 DHCP server
Port 68 — IPv4 DHCP client

Checking What's Listening

To see if a DHCPv6 server is listening on your system:

# Linux/macOS
sudo lsof -i :547
sudo netstat -uln | grep 547

# Windows
netstat -an | findstr :547

Active port 547 listeners usually indicate a DHCPv6 server or relay agent running on that machine.