IPv4 & IPv6 Multicast Basics

Understanding multicast addressing, scopes, well-known groups, and local subnet limitations.

What is Multicast?

Multicast allows one sender to transmit data to multiple receivers simultaneously. Unlike broadcast (everyone) or unicast (one specific recipient), multicast sends to a specific group of interested receivers. This is more efficient than sending individual unicast packets to each recipient, especially for streaming media, software updates, or real-time communications.

IPv4 Multicast

Range: 224.0.0.0/4 (224.0.0.0 to 239.255.255.255)

Local Network Control Block
Range: 224.0.0.0/24
Description: Well-known multicast addresses for protocols
Scope: Local subnet only
Examples:
224.0.0.1 - All Systems
224.0.0.2 - All Routers
224.0.0.5 - OSPF All SPF Routers
224.0.0.6 - OSPF All DR Routers
224.0.0.9 - RIP Version 2
224.0.0.22 - IGMP
224.0.0.251 - mDNS
224.0.0.252 - LLMNR
Internetwork Control Block
Range: 224.0.1.0/24
Description: Internet-wide control protocols
Scope: Global
Examples:
224.0.1.1 - NTP
224.0.1.60 - DHCP Server/Relay
AD-HOC Block I
Range: 224.1.0.0 to 224.1.255.255
Description: Transient use for applications
Scope: Variable
Examples:
Application-specific addresses
SDP/SAP Block
Range: 224.2.0.0 to 224.2.255.255
Description: Session Directory Protocol
Scope: Global
Examples:
Session announcements
Organization Local Scope
Range: 239.0.0.0/8
Description: Private multicast addresses
Scope: Organization-wide
Examples:
Private applications, enterprise use

IPv6 Multicast

Range: ff00::/8 (all addresses starting with ff)

Address Structure

Format: ff[flags][scope]::/16

Flag Bits
0 - Reserved (always 0)
0 - Reserved (always 0)
0 - Reserved (always 0)
T - 0=Permanent, 1=Temporary
Scope Values
0 - Reserved
1 - Interface-Local
2 - Link-Local
3 - Reserved
4 - Admin-Local
5 - Site-Local
8 - Organization-Local
e - Global

Well-Known IPv6 Multicast Addresses

AddressNameDescription
ff01::1All Nodes (Interface-Local)All nodes on the same interface
ff02::1All Nodes (Link-Local)All nodes on the local network segment
ff02::2All Routers (Link-Local)All routers on the local network segment
ff02::5OSPFv3 All SPF RoutersAll OSPF routers
ff02::6OSPFv3 All DR RoutersOSPF designated routers
ff02::9RIPng RoutersRIP next generation routers
ff02::fbmDNSv6Multicast DNS over IPv6
ff05::2All Routers (Site-Local)All routers within the site

Common Protocol Multicast Addresses

ProtocolIPv4IPv6Purpose
OSPF224.0.0.5, 224.0.0.6ff02::5, ff02::6Routing protocol communication
RIP224.0.0.9ff02::9Routing updates
DHCP224.0.0.252ff02::1:2Configuration relay
mDNS224.0.0.251ff02::fbZero-configuration networking
LLMNR224.0.0.252ff02::1:3Link-local name resolution
SSDP239.255.255.250ff0x::cUPnP device discovery

Important Limitations

Local Subnet Only Caveat

Many multicast addresses are designed for local subnet use only

  • 224.0.0.x addresses don't cross router boundaries by default
  • ff02:: addresses are link-local scope in IPv6
  • Routers need multicast routing (PIM, IGMP) to forward between subnets
  • Without multicast routing, traffic stays on the local segment
  • This is intentional for protocol efficiency and security
IGMP/MLD Requirements

Devices must signal interest in multicast groups

  • IGMP (IPv4) or MLD (IPv6) tells routers which groups are wanted
  • Without IGMP/MLD, routers may drop multicast traffic
  • Switches need IGMP snooping to avoid flooding
  • Managed switches should have multicast features enabled
Firewall Considerations

Firewalls often block multicast by default

  • Corporate firewalls frequently block multicast ranges
  • Home routers may not forward multicast between VLANs
  • VPN tunnels typically don't carry multicast traffic
  • Cloud environments often don't support multicast

Troubleshooting Common Issues

Multicast not working across VLANs
Common Causes:
  • No multicast routing configured
  • IGMP snooping disabled
  • Firewall blocking
Solutions:
  • Enable PIM on routers
  • Configure IGMP snooping
  • Allow multicast ranges in firewall
High multicast traffic flooding network
Common Causes:
  • No IGMP snooping
  • Switches flooding multicast
  • Rogue applications
Solutions:
  • Enable IGMP snooping on switches
  • Monitor multicast sources
  • Implement multicast rate limiting
Devices not receiving multicast
Common Causes:
  • Not joined to group
  • IGMP queries not working
  • Wrong scope
Solutions:
  • Verify group membership
  • Check IGMP querier
  • Use correct multicast scope

Best Practices

  • Use appropriate multicast scopes (link-local vs site-local vs global)
  • Enable IGMP snooping on managed switches
  • Configure multicast routing (PIM) only where needed
  • Monitor multicast traffic to prevent network flooding
  • Use organization-local ranges (239.x.x.x) for private applications
  • Test multicast applications in isolated environments first
  • Document multicast group assignments to avoid conflicts

Quick Reference

IPv4 Quick List
224.0.0.1 - All systems on subnet
224.0.0.2 - All routers on subnet
239.x.x.x - Private/organization use
IPv6 Quick List
ff02::1 - All nodes on link
ff02::2 - All routers on link
ff0x::... - Various scopes (x = scope)
Key Remember
Most multicast addresses are designed for local subnet use only. Without proper multicast routing configuration, traffic won't cross router boundaries.