Magic Damage Reduced

Diablo II	Notation	Basic	Class	Skill	Item	Ally	Quest	Area	Monster	Tale

Crushing Blow	Deadly Strike	Enhanced	Minimum	Maximum	Adds Damage	Damage +	% Damage
to Demons	to Undead	Magic	Fire	Lightning	Cold	Poison	Skill Damage
ATD/ATLD	DR/MDR	Resistance	Maximum	Enemy Resist	Sunder	Immune	Absorb

Magic Damage Reduced applies to any magic and elemental (fire, lightning and cold) damage remaining after Cyclone Armor and then Energy Shield have been applied. MDR applies to each separately, so an attack applying 20 magic, 20 fire, 20 lightning and 20 cold damage would apply no damage after MDR 20 was applied. [1]

Effectiveness is reduced when applied to magic or elemental damage applied by the following missile attacks:

Damage Rate
41/1024 (~1/25)	82/1024 (~2/25)	205/1024 (~1/5)
Arctic Blast Armageddon fire Fire Wall Firestorm Immolation Arrow pyre Meteor pyre Molten Boulder fire Catapult meteor pyre Countess fire wall Diablo armageddon fire Diablo firestorm Vampire fire wall Vampire meteor pyre	Inferno Wake of Inferno Diablo lightning Fetish Shaman inferno Megademon inferno Salamander fire cloud	Wake of Fire

[2]

When MDR completely negates magic and elemental damage, it does not matter if Lower Resist, Conviction or -% Enemy Resistance then reduce resistance, since there is no damage to which resistance can be applied (although resistance will still apply to cold length or freeze length).
A combination of resistance and Absorb is generally more effective at significantly reducing or even negating magic and elemental damage.
When an attack applies magic or elemental damage and physical damage, and MDR > magic or elemental damage, the excess is applied to physical damage after Damage Resist or Damage Reduced by % has been applied. [3] [4]
For example, an attack applying 100 physical damage and 50 fire, 50 lightning and 50 cold damage to a target with MDR 100 and DR 50%, would apply no fire (50-100 = -50), lightning (-50) or cold (-50) damage and 50 physical damage before excess MDR (-150 in total) applied: this would result in no (50-150 < 0) damage.

Item

Helm

Value	Source	Min Rlvl	Ladder
7	Mal Rune in socket	49	No

Value	Source	Min Rlvl	Ladder
(1-2)	Safety helm	13	No
2	Howltusk Great Helm	25	No
3	Radiance helm	27	No
3	Natalya's Totem Grim Helm	59	No
10	Hwanin's Splendor Grand Crown	45	No
(10-15)	Vampire Gaze Grim Helm	41	No

Circlet

Value	Source	Min Rlvl	Min Alvl
1	Circlet of Warding	5	7
2	Circlet of the Sentinel	12	18
3	Circlet of Guarding	19	26
(4-6)	Circlet of Negation	35	42
2	Circlet of the Sentinel	18	24
3	Circlet of Guarding	24	32
(4-6)	Circlet of Negation	33	41

Amulet

Value	Source	Min Rlvl	Min Alvl
1	Amulet of Warding	5	7
2	Amulet of the Sentinel	12	18
3	Amulet of Guarding	19	26
(4-6)	Amulet of Negation	35	42

Value	Source	Min Rlvl	Ladder
1	Tancred's Weird Amulet	20	No
(1-2)	Safety Amulet	13	No
10	Crescent Moon Amulet	50	No
(8-12)	Entropy Locket Amulet	54

Body armor

Value	Source	Min Rlvl	Min Alvl
1	Body armor of Warding	5	7
2	Body armor of the Sentinel	18	24
3	Body armor of Guarding	24	32
(4-6)	Body armor of Negation	33	41

Value	Source	Min Rlvl	Ladder
7	Mal Rune in socket	49	No

Value	Source	Min Rlvl	Ladder
(1-2)	Safety body armor	13	No
1	Iceblink Splint Mail	22	No
2	Berserker's Hauberk Splint Mail	3	No
2	Isenhart's Case Breast Plate	8	No
3	Greyform Quilted Armor	7	No
3	Stealth body armor	17	No
7	Rain body armor	49	No
(6-10)	Que-Hegan's Wisdom Mage Plate	51	No
10	Skullder's Ire Russet Armor	42	No
(7-11)	The Spirit Shroud Ghost Armor	28	No
(5-12)	M'avina's Embrace Kraken Shell	70	No
(9-13)	Skin of the Vipermagi Serpentskin Armor	29	No
15	Tal Rasha's Guardianship Lacquered Plate	71	No
(10-16)	Iron Pelt Trellised Armor	33	No
17	Prudence body armor	49	No
(15-20)	The Gladiator's Bane Wire Fleece	85	No

Axe

Value	Source	Min Rlvl	Ladder
(12-15)	Spellsteel Bearded Axe	39	No

Bow

Value	Source	Min Rlvl	Ladder
(9-13)	Magewrath Rune Bow	43	No

Club

Value	Source	Min Rlvl	Ladder
2	Stoutnail Spiked Club	5	No
2	Black club	35	No

Hammer

Value	Source	Min Rlvl	Ladder
2	Black hammer	35	No

Mace

Value	Source	Min Rlvl	Ladder
2	Black mace	35	No
(9-12)	Moonfall Jagged Star	42	No

Orb

Value	Source	Min Rlvl	Min Alvl
1	Orb of Warding	5	7
2	Orb of the Sentinel	12	18
3	Orb of Guarding	19	26
(4-6)	Orb of Negation	35	42

Scepter

Value	Source	Min Rlvl	Ladder
1	Rusthandle Grand Scepter	17	No

Spear

Value	Source	Min Rlvl	Ladder
(1-2)	Safety spear	13	No

Staff

Value	Source	Min Rlvl	Ladder
7	Memory staff	37	No
(5-8)	Ondal's Wisdom Elder Staff	66	No
15	Razorswitch Jo Staff	28	No

Sword

Value	Source	Min Rlvl	Ladder
3	The Patriarch Great Sword	29	No
(7-12)	Ginther's Rift Dimensional Blade	37	No

Wand

Value	Source	Min Rlvl	Ladder
4	White wand	35	No

Shield

Value	Source	Min Rlvl	Min Alvl
1	Shield of Warding	5	7
2	Shield of the Sentinel	18	24
3	Shield of Guarding	24	32
(4-6)	Shield of Negation	33	41

Value	Source	Min Rlvl	Ladder
7	Mal Rune in socket	49	No

Value	Source	Min Rlvl	Ladder
(1-2)	Safety shield	13	No
1	Stormguild Large Shield	13	No
2	The Ward Gothic Shield	26	No
5	Bverrit Keep Tower Shield	19	No
7	Sanctuary shield	49	No
(14-18)	Gerke's Sanctuary Pavise	44	No

Auric shield

Value	Source	Min Rlvl	Ladder
(5-9)	Alma Negra Sacred Rondache	77	No

Ring

Value	Source	Min Rlvl	Ladder
(1-2)	Safety Ring	13	No
3	Nagelring Ring	7	No
(12-15)	Dwarf Star Ring	45	No

Value	Source	Min Rlvl	Min Alvl
1	Ring of Warding	5	7
2	Ring of the Sentinel	12	18

Belt

Value	Source	Min Rlvl	Ladder
(1-2)	Safety belt	13	No
(10-15)	String of Ears Demonhide Sash	29	No

Boots

Value	Source	Min Rlvl	Ladder
1	Goblin Toe Light Plated Boots	22	No
(1-2)	Safety boots	13	No

Gloves

Value	Source	Min Rlvl	Ladder
(1-2)	Safety gloves	13	No

Set bonus

Value	Source	Min Rlvl	Ladder
3	Cathan's Traps (Full)	11	No
15	Natalya's Odium (2 items)	59	No
10	Immortal King (Full)	76	No

Reference

A few random questions (D2 Technical)
PDR/MDR Overflow Mechanic (D2 Technical)
Gustafsson, Tommi, Magic Damage Reduction Modification vs, Frame-Based Attacks (Internet Archive)