Calculator
Example Data Table
| Multicast IPv4 | Lower 23 Bits | Mapped MAC Address | Usage Note |
|---|---|---|---|
| 224.0.0.1 | 0x000001 | 01:00:5E:00:00:01 | All hosts group on local network |
| 224.0.0.251 | 0x0000FB | 01:00:5E:00:00:FB | Commonly used by multicast DNS |
| 239.192.1.10 | 0x40010A | 01:00:5E:40:01:0A | Administratively scoped multicast example |
| 239.255.255.250 | 0x7FFFFA | 01:00:5E:7F:FF:FA | Often used by discovery traffic |
Formula Used
Valid multicast IPv4 range: 224.0.0.0 through 239.255.255.255.
Fixed Ethernet multicast prefix: 01:00:5E.
Core mapping: MAC = 01:00:5E + lower 23 bits of the multicast IPv4 address.
Bit mask: lower23 = IPv4_decimal AND 0x7FFFFF.
Fourth MAC byte: (lower23 >> 16) AND 0x7F.
Fifth MAC byte: (lower23 >> 8) AND 0xFF.
Sixth MAC byte: lower23 AND 0xFF.
Collision rule: Five multicast IP bits are discarded. So 32 multicast IPv4 addresses can share one Ethernet MAC address.
How to Use This Calculator
- Enter one multicast IPv4 address in the single address field.
- Optionally paste many addresses in the batch field.
- Select the MAC output style, such as colon, hyphen, dot, or plain.
- Choose uppercase or lowercase output.
- Enable alias examples when you want collision details.
- Press the convert button.
- Review the MAC address, binary steps, and lower 23-bit value.
- Use CSV or PDF buttons to save the report.
Understanding Multicast IP to MAC Conversion
What This Tool Does
This calculator converts an IPv4 multicast address into its Ethernet multicast MAC address. Multicast addressing helps one sender deliver traffic to many receivers. It is common in streaming, routing, discovery, and service announcement systems. The tool checks the address before calculating the result. Only addresses from 224.0.0.0 to 239.255.255.255 are accepted.
Why the Mapping Matters
Ethernet frames need MAC addresses. IP packets need IP addresses. When multicast traffic crosses an Ethernet network, the multicast IP must map to a multicast MAC. Network cards and switches use this MAC value to filter traffic. Correct mapping helps with packet captures, firewall rules, switch checks, and troubleshooting.
How the Bits Are Used
The Ethernet multicast prefix is fixed as 01:00:5E. The final three bytes come from the lower 23 bits of the multicast IP address. The highest bit of the fourth MAC byte is always zero. This means the mapping does not keep every IP bit. Five bits are dropped during the conversion.
Why Collisions Can Happen
IPv4 multicast uses 28 variable group bits. Ethernet mapping keeps only 23 of those bits. Because five bits are lost, 32 multicast IP groups can point to the same MAC. This does not always break traffic. Still, it can explain unexpected packets in captures. It can also explain why a network interface receives traffic for nearby multicast groups.
Practical Network Use
Use this page when checking IPTV groups, routing protocols, lab traffic, or discovery messages. It is useful before writing switch filters. It also helps when comparing packet analyzer output with configured multicast groups. Batch mode makes audits faster. The downloadable reports help document findings for teams. The chart gives a quick visual view of the IP octets and MAC bytes.
FAQs
1. What is a multicast IPv4 address?
A multicast IPv4 address is an address from 224.0.0.0 to 239.255.255.255. It represents a group, not one host. Devices join that group to receive matching multicast traffic.
2. What MAC prefix is used for IPv4 multicast?
IPv4 multicast Ethernet frames use the fixed prefix 01:00:5E. The remaining MAC bytes are built from the lower 23 bits of the multicast IPv4 address.
3. Why are only 23 IP bits used?
The Ethernet multicast mapping reserves 25 fixed MAC bits. That leaves 23 bits for the group value. IPv4 multicast has more group bits, so five bits are discarded.
4. Can two multicast IP addresses share one MAC?
Yes. Since five IP bits are lost, 32 multicast IPv4 addresses can map to the same Ethernet MAC address. This is called an address mapping collision.
5. Is 224.0.0.1 a valid input?
Yes. 224.0.0.1 is a valid multicast IPv4 address. It maps to 01:00:5E:00:00:01 using the standard Ethernet multicast mapping method.
6. Does this calculator support IPv6 multicast?
No. This calculator is designed for IPv4 multicast addresses only. IPv6 multicast uses a different address structure and a different Ethernet MAC mapping method.
7. Why is my unicast IP rejected?
Unicast IPv4 addresses are outside the multicast range. This tool accepts only 224.0.0.0 through 239.255.255.255 because only those addresses use this mapping.
8. What is the use of CSV and PDF export?
CSV export helps with spreadsheets and bulk checks. PDF export is useful for reports, audits, training notes, network documentation, and troubleshooting records.