In order to melt a network solid, a relatively large amount of heat is needed, sufficient to break the covalent bonds that hold the entire solid together. Network solids are crystalline solids that are essentially giant molecules with repeating structures, such as diamonds or quartz crystals.
Network solids are one of four types of crystalline solids. The others are ionic solids, metallic solids and molecular solids. Ionic solids, such as crystals of table salt, are the most similar to network solids, in that they are repeating blocks of atoms all bonded to each other in the same way. The difference is that, in ionic solids, these bonds are ionic rather than covalent. Both types of solids tend to be hard and brittle, with a high melting point.
Molecular solids, such as solid carbon dioxide or dry ice, are held together with intermolecular forces rather than chemical bonds. These include hydrogen bonds and dipole-dipole forces. They tend to have low melting points. Metallic solids are like network and ionic solids in that they do not have distinct molecules, but the bonds between the constituent atoms are metallic bonds. Metallic solids have variable hardnesses and melting points, and are generally good conductors of electricity.